API: Allow to set or update a merge-request's milestone

_Originally opened at !2107 by @ skakirill._

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See merge request !2755

Don't show fork button if user can't fork the project.

Fixes #13369 

See merge request !2793

Load all blob data when highlighting content for git-blame.

Fixes #13365

See merge request !2792

Add a new image in the markdown help page

Previously we were linking directly from the public/
directory. While this worked for the GitLab help page,
it was not working for doc.gitlab.com. Adding an image
in a relative directory and linking from there serves
both ends.

Fixes #13233

See merge request !2790

Previously we were linking directly from the public/
directory. While this worked for the GitLab help page,
it was not working for doc.gitlab.com. Adding an image
in a relative directory and linking from there serves
both ends.

[ci skip]

Remove sqlite check in raketask

This is a relic from 4 major releases ago. Time to remove it.

See merge request !2777

Fixed "Docker Fundamentals" link in GitLab Documentation.

I noticed that the *Docker Fundamentals* link in the *Using Docker Images* part of the GitLab Documentation is broken.

Screenshot:
![broken-link](/uploads/8d2fc782b2f5d4f3316a631f801cd3c7/broken-link.png)

Changing it to "https://docs.docker.com/" should fix the problem.

See merge request !2786

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