Add merge_requests/:merge_request_id/closes_issues

_Originally opened at !2660 by @Schniz._

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Added `/api/v3/projects/:id/merge_requests/:merge_request_id/closes_issues` route in the API for getting the issues that should be closed when a merge request merged successfully

See merge request !2779

Use caching

This enables to use built-in caching feature instead of using the `/cache` folder.

See merge request !2747

Fix CI build status badge

Closes #5570, #4935, #4163

See merge request !2704

We keep this only for backwards compatibility with projects that have
been migrated from GitLab CI. New project badge will go elsewhere.

Added some documentation for it

Added to changelog

Added curl example and an attribute table

Moved the api route definition from "lib/api/issues.rb" to "lib/api/merge_requests.rb"

Fixed the attributes and changed the documentation to be at "merge_requests.md" too

Changed generic titles to more specific titles

added an underscore

added tests. it depends on a newer version of gitlab-test project
I'm doing a  since I need to add a branch to the `gitlab-test` repo

removed the before

using 'iid' instead of 'id' in the description to reference the issues. that makes the tests pass

Removed the 'closes-issues' key from test_env. so it should pass the tests

Moved the two initializations to the describe block

Changed the changelog

Replaces "Create merge request" link with one to the MR when one exists

Fixes #3716

* Without MR:

![before-1](/uploads/abcb2eb5bb5927a83c2f1700528f852c/before-1.png)

* With MR:

![after-1](/uploads/9020b871e1aa08d3e5a45359336ef0f2/after-1.png)

* Without MR:

![before-2](/uploads/bb3bd1a74efc193ad31af68f1e04ef5f/before-2.png)

* With MR:

![after-2](/uploads/789d32aef12f39c605e665d0c4bc9fbe/after-2.png)

* Without MR:

![before-3](/uploads/a9aff7a11eb1069be24c37a5455be809/before-3.png)

* With MR:

![after-3](/uploads/48fe81ea50657fe5b2de0d25260d8e4b/after-3.png)

See merge request !2670

Add assignee data to Issuables' hook_data

Originally opened at !1633 by @brammeleman.

Fixes #3047. Fixes #2475.

See merge request !2724

Actually use the `skip_merges` option in Repository#commits

Originally opened at !2067 by @cwq1913.

See merge request !2750

Fix builds API formatting

This MR is fixing formatting for builds API documentation.

See merge request !2775

[ci skip]

[ci skip]

Add documentation on relative URL support

closes #12773

See merge request !2770

[ci skip]

Updated install/update guides for 8.5



See merge request !2766

[ci skip]

Add ajax filtering to all paged project lists. Fixes #13182



See merge request !2728

Ensure link for README blobs includes the full path

Closes #13254

See merge request !2774