Ability to bulk assign issues to author of merge request

## What does this MR do?
Provides a link to auto-assign issues to the author of a merge request, when they are mentioned as being closed by the MR.

## Are there points in the code the reviewer needs to double check?


## Why was this MR needed?
To help avoid working on a MR without having assigned related issues to self

## What are the relevant issue numbers?
Fixes #18876 

## Screenshots (if relevant)
![ScreenShot-P216](/uploads/1af5e71a0a0ff0a60c5d7b54c0e09d9c/ScreenShot-P216.png)

## Tasks
- [x] Refactor or move away from using `BulkUpdateService`
- [x] ~~Consider alternate link message when only a subset of issues will be assigned~~
- [x] Minimize repeated calls to expensive `closes_issues` method
- [x] Move away from using inflector for pluralization and fix flash message
- [x] Change auth `before_action` and fallback to error flash message
- [x] Shouldn't overwrite current assignee if one exists

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5725

Make guests unable to view MRs

## What does this MR do?

Make guests unable to view MRs. This also fixes a bug when a non-member user could get notification if mentioned (in private project, it's OK for public project).

Now if you are a guest and you will be mentioned in one of the MRs you won't get a notification. 

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/1410

See merge request !6673

Build instead create in label_link model spec

## What does this MR do?
This MR improves spec for label link model.

## Are there points in the code the reviewer needs to double check?
No

## Why was this MR needed?
Because for check validations, we can use build instead create a record in the database. 

## Screenshots (if relevant)
No

## Does this MR meet the acceptance criteria?

- n/a [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- n/a [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- n/a API support added
- Tests
  - n/a Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- n/a [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !6792

Remove empty describe block on key spec model

## What does this MR do?
Removes unnecessary describe in test

## Are there points in the code the reviewer needs to double check?
No

## Why was this MR needed?
It describe block not needed.

## Screenshots (if relevant)
No

## Does this MR meet the acceptance criteria?

- n/a [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- n/a [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- n/a API support added
- Tests
  - n/a Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
No

See merge request !6793

Move some CHANGELOG entries to the 8.13.0 part

Spotted while reviewing https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/789/diffs#diff-1

[ci skip]

See merge request !6809

Spotted while reviewing
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/789/diffs#diff-1.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Move operations/ to new location

## What does this MR do?

Move operations/ to new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [ ] Make sure internal links pointing to the document in question are not broken.
- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [ ] If working on CE, submit an MR to EE with the changes as well.

See merge request !6519

Remove duplicate CHANGELOG entry

mail_room was bumped for 8.12.5.

See merge request !6806

[ci skip]

Allow browsing branches that end with '.atom'

## What does this MR do?

1. Simplify the regex capture in the routing for the CommitsController
   to not exclude the '.atom' suffix. That's a perfectly valid git
   branch name, so we shouldn't blow up if we get it.
2. Because Rails now can't automatically detect the request format, add
   some code to do so in `ExtractPath` when there is no path. This means
   that, given branches 'foo' and 'foo.atom', the Atom feed for the
   former is unroutable. To fix this: don't do that! Give the branches
   different names!

## Why was this MR needed?

Creating a branch or tag name ending in '.atom' would cause some 500s on that repo.

## What are the relevant issue numbers?

Closes #21955. Related to !5994.

See merge request !6750

Move health check docs under user/admin_area/monitoring



See merge request !6807

[ci skip]

Add examples of fake tokens to be used in docs



See merge request !6805

Move monitoring/ to new location

## What does this MR do?

Move monitoring/ to new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [ ] Make sure internal links pointing to the document in question are not broken.
- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [ ] If working on CE, submit an MR to EE with the changes as well.

See merge request !6518

[ci skip]

Move reply by email docs to a new location

## What does this MR do?

Move reply by email docs to a new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [ ] Make sure internal links pointing to the document in question are not broken.
- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [ ] If working on CE, submit an MR to EE with the changes as well.

See merge request !6517

Update pipeline graph styles to match mockup

#### What does this MR do?
* Updates CSS for pipeline graph to match mockup 
* Adds JS to dynamically change line widths based on number of builds

#### Screenshots (if relevant)
![Screen_Shot_2016-10-04_at_2.06.24_PM](/uploads/362378ca23e18a3cc59185eafab9f9ac/Screen_Shot_2016-10-04_at_2.06.24_PM.png)

#### What are the relevant issue numbers?
Closes #21604

See merge request !6320

We need to do two things to support this:

1. Simplify the regex capture in the routing for the CommitsController
   to not exclude the '.atom' suffix. That's a perfectly valid git
   branch name, so we shouldn't blow up if we get it.
2. Because Rails now can't automatically detect the request format, add
   some code to do so in `ExtractPath` when there is no path. This means
   that, given branches 'foo' and 'foo.atom', the Atom feed for the
   former is unroutable. To fix this: don't do that! Give the branches
   different names!

Add SubGit to the tools of SVN migration

Originally from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6549.

See merge request !6803

[ci skip]

Document the new CI_DEBUG_TRACE variable a feature that will
be introduced in GitLab Runner v1.7

Related to https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/339

See merge request !6700

[ci skip]

Fix Error 500 when viewing old merge requests with bad diff data

Customers running old versions of GitLab may have MergeRequestDiffs with the text ["--broken diff"] due to text generated by gitlab_git 1.0.3.
To avoid the Error 500, verify that each element is a type that gitlab_git will accept before attempting to create a DiffCollection.
    
Closes #20776

See merge request !6754

Add a new gitlab:users:clear_all_authentication_tokens task

## What are the relevant issue numbers?

Part of #22537.

See merge request !6745

Update mail_room gem

## What does this MR do?

Upgrades the mail_room gem from 0.8.0 to 0.8.1.

## Why was this MR needed?

mail_room throws an exception if the watching script joins a undefined thread. Release 0.8.1 adds a check to ensure that the thread exists before calling join.

Fixes gitlab-org/omnibus-gitlab#1619.

See merge request !6714

Remove redundant images from GitLab basics docs



See merge request !6799

Add an example for testing a phoenix application with Gitlab CI

_Originally opened at !5074 by @nahtnam._

- - -

## What does this MR do?

Add an example on how to setup testing on phoenix 

## Are there points in the code the reviewer needs to double check?

Grammar and formatting

## Why was this MR needed?

https://twitter.com/gitlab/status/749898132008337408

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6794

Replace images in GitLab basics docs



See merge request !6786

[ci skip]

Precalculate trending projects

## What does this MR do?

This MR changes the trending projects code so that data is precalculated, removing the need for any complex caching mechanisms. 

## Why was this MR needed?

Caching of trending data didn't work properly, still leading to queries. Using caching in general would be very hard due to users being able to apply custom filters to the list of trending projects.

See merge request !6749

Allows empty merge requests for a workflow described in gitlab-org/gitlab-ce#14606

Closes gitlab-org/gitlab-ce#14606

See merge request !6384

[ci skip]

Add `robots.txt` to the list of reserved namespaces

See #22759

See merge request !6789