api: expose web_url for project entities

## What does this MR do?

Adds a `web_url` field to project snippet, issues, and merge request API returned objects.

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

Tests. Are they sufficient? (I'm working on getting them to pass.)

## Why was this MR needed?

Creating the URL for an MR or issue from the MR is tedious in API libraries; it's easy for Gitlab to just provide it.

## What are the relevant issue numbers?

N/A

## Screenshots (if relevant)

N/A

## 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 !5631

Update merge_requests.md with a simpler way to check out a merge request

## What does this MR do?

This MR updates `merge_requests.md` with a simpler way to check out a merge request 

The previous way was put under the "By modifying `.git/config` for a given repository" section. The disadvantage of the old way is that one has to modify `.git/config` for each repository that one uses. The new way was put under the "By adding a git alias" section. One has to create this git alias once, and then it will work from any repository, without having to modify `.git/config` over and over again.

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

One should double check that the link provided by the "Check out branch" button in a MR, i.e. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/workflow/merge_requests.md#checkout-merge-requests-locally, still works. I think it will, since I didn't modify the "Checkout merge requests locally" section name.

## Why was this MR needed?

This MR was needed because the old way of checking out a merge request locally requires to modify `.git/config` in all repositories that one has on a computer, which is a tedious task. The new way just requires adding a simple git alias, once. And the `git mr` command then works for all repositories.

## What are the relevant issue numbers?

I haven't created an issue for this.

## Screenshots (if relevant)

N/A

## 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)
- [ ] API support added (N/A)
- Tests
  - [ ] Added for this feature/bug (N/A)
  - [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 !5944

Api project hook wiki page events

## What does this MR do?

Updates project hook API for wiki page events. Also updates docs and tests for forgotten hook fields.

See merge request !5969

last_push_event widget considers fork events on the main project

Push events on the fork of the current project are considered to be shown besides the push events on the current project. Before we just consider fork events if exists a fork so we miss current project events if the current user has a fork.

Closes #20254

See merge request !5978

Issue created at docs

## What does this MR do?

Clarifies `created_at` for creating issues via the API.

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

No.

## Why was this MR needed?

Docs were unclear.

## What are the relevant issue numbers?

Closes #18458 and closes #19863.

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [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 !5967

Add tests coverage report badge to README

## What does this MR do?

This MR adds tests coverage badge to project's README.

The job it reads coverage value from is `coverage`, and it is configured in out CI test suite to calculate compound coverage from all test jobs running in parallel.

This will pick the latest successful pipeline to get `coverage` job and coverage value.

For now, this will report test coverage only for backend test suite (Ruby).

See merge request !5925

Update Ruby version to 2.3 in Requirements

According to the [GitLab 8.11 release notes](https://about.gitlab.com/2016/08/22/gitlab-8-11-released/), Ruby 2.3 is now supported and recommended. Update the documentation accordingly.

See merge request !5965

This allows web hooks to have a URL back to entities without having to
generate it themselves.

See also #19863.

Fixes #18458.

Disable commenting on unrelatable diff line

## What does this MR do?

Fixes a bug where users can comment on diff lines that can't be commented on and attached to.

This is the case for unfolded lines and lines that appear as snippets in the discussion tab.

**!5864 should be merged before this MR.**

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

😕 

## Why was this MR needed?

Comments were getting lost on the discussion feed, unable to find their related diff.

## What are the relevant issue numbers?

Closes #20633.

## Screenshots (if relevant)

![diffs-comment-fix](/uploads/f902b687fda75492e38397f9705283e0/diffs-comment-fix.mp4)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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 [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5681

Fix file links on project page Files view

## What does this MR do?

Fix the project show page when the default view is set to Files so that the links work.

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

I can't figure out what caused this to break 😕 

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-com/support-forum/issues/959.

## 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) (N/A)
- [x] API support added (N/A)
- Tests
  - [x] Added for this feature/bug
  - [ ] 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 !5933

Now the alias should be used as:

    git mr upstream 5

To check out a MR number 5 from the upstream remote, into a `mr-upstream-5`
local branch.

Also clarify that the `5` in the merge request is the number shown in GitLab.

Fix Spinach branches spec

I broke master by adding a branch to gitlab-test for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5961, which put the branch in this scenario off the first page 😆 

See merge request !5964

Also check if Akismet is enabled, before showing the `Submit as spam` button.

Fixes #21229

See merge request !5948

Fix incorrect "stopped impersonation" log message

Minor bug: Usernames were swapped in the log message.

Closes #21015

See merge request !5949

Fix project namespace links

#### What does this MR do?
Fixes pipeline links

#### Why was this MR needed?
Links went to a 404 on fork MR pipelines tab

#### What are the relevant issue numbers?
Closes #21107

See merge request !5912

entities: make Environment inherit EnvironmentBasic

## What does this MR do?

Makes the `Environment` entity inherit from `EnvironmentBasic`

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

No.

## Why was this MR needed?

Just a code simplification.

## What are the relevant issue numbers?

N/A

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [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 !5959

Change merge_error column from string to text type

See merge request !5950

Fix typo in gitlab_flow.md

## What does this MR do?

Cross MR from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/674

See merge request !5960

In some cases, the artificial 255-character limit would cause an error in
MergeWorker.

Closes #20593

Fixed enter key in search input not working

## What does this MR do?

Fixes a bug where the enter key wouldn't search in the top nav search input

Closes #20627 

See merge request !5888

Closes #20627