Start to document how to code for CE with EE in mind

Closes #23740

[ci skip]

See merge request !7248

Signed-off-by: Rémy Coutable <remy@rymai.me>

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Signed-off-by: Rémy Coutable <remy@rymai.me>

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

Include author in assignee dropdown search

## What does this MR do?

Previously when you searched for the author in the assignee dropdown (if they aren't in the project) it did not actually include the author. This changes that so that when you search for the author it will correctly include the user.

## Screenshots (if relevant)

![Screen_Shot_2016-11-17_at_11.14.53](/uploads/5a13475d8ded459fd3b1ba6570897665/Screen_Shot_2016-11-17_at_11.14.53.png)

## What are the relevant issue numbers?

Closes #22905

See merge request !7526

500 error on project show when user is not logged in and project is still empty

## What does this MR do?

Aims to fix the 500 error when the project is empty and the user is not logged in and tries to access project#show

## Screenshots (if relevant)
When the project is empty and the user is not logged in we default to the empty project partial instead of readme.

![Screen_Shot_2016-11-11_at_22.54.21](/uploads/3d87e65195376c85d3e515e6d5a9a850/Screen_Shot_2016-11-11_at_22.54.21.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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 [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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #23990

See merge request !7376

JiraService: simplify url generation

Fixes #24085

See merge request !7307

Mention Git strategy none

See merge request !7530

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adds fix for security issue when annonymous user does not have access to repository we now display the activity feed instead of the readme

Highlight first user autocomplete option

See merge request !7520

Add documentation for Review Apps

## What does this MR do?

Add docs for review apps.

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !6982

Allows configuration of idle_timeout for incoming email.

https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1087

See merge request !7423

Improvements to Redis HA docs

Following https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6471

See merge request !7502

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Remove empty db fixtures file

## What does this MR do?
Removes empty fixtures file

See merge request !7519

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Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

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Refactor environments documentation

## What does this MR do?

Refactor environments docs.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23484

See merge request !7107

[ci skip]

When searching for the author in the assignee dropdown it now correctly returns the user

Closes #22905

[ci skip]

[ci skip]

Stop environment when branch is deleted

## What does this MR do?

This MR adds a environment teardown service, that is called when user deletes a branch. This most often happens when merge requests is merged.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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

## What are the relevant issue numbers?

Closes #23218

See merge request !7355

Update CONTRIBUTING.md - Adding info about issues within a milestone

## What does this MR do?
Adds copy regarding issues within the current milestone to the contribution guide.

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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)

cc @dzaporozhets @awhildy

See merge request !7516

Revert "Merge branch '22680-unlabel-limit-autocomplete-to-selected-items' into 'master'"

There were some issues with the initial implementation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796#note_18557076

This reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6796

See merge request !7521

Remove ToC and fix headings in Markdown docs

See merge request !7523

Adds hash anchors to diff_files to make possible linking exact file in commit

Closes #24010

See merge request !7298

Signed-off-by: Rémy Coutable <remy@rymai.me>