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

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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

Closes #13254

Add notice about variables in build log

Related to: https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1942

[ci skip]

See merge request !2761

Prevent rasterization artifacts in the logo, and simplify markup.

 - Remove sketch-namespaced attributes.
 - Remove unused groups, and cancel out unnecessary transforms.
 - Use overlap to avoid rasterization artifacts in the logo.
 - Remove viewBox parameter and define paths in their final form.

See merge request !2741

[ci skip]

Freeze constants in Gitlab::Git

See merge request !1730

Smarter flushing of branch statistics caches

This basically ensures we only flush caches of branches whenever we really have to. See commit c514f8b850219cd3e5526e73e1d00e6729e2b466 for the details.

cc @joshfng @rspeicher 

See merge request !2769

Change default smpt port to 465 (default smtps port)

Instead of flushing the behind/ahead counts for all branches upon every
push we now only flush the cache of branches that actually need to have
these statistics recalculated. There are now basically 2 scenarios and
their effects:

1. A user pushes a commit to the default branch, this results in the
   cache being flushed for all branches.
2. A user pushes to a non default branch, this results in _only_ the
   cache for that branch being flushed.

The existing code (Repository#expire_cache) remains backwards compatible
with the previous behaviour, the new behaviour is only applied when a
branch name is passed as an argument. This ensures that when for example
a project is deleted the cache for all branches is flushed.

Add index for snippet's updated_at

@DouweM We are ordering by `updated_at` but there is no index.

cc @yorickpeterse 

See merge request !2768

Expose MergeRequest#merge_status

_Originally opened at !2478 by @develop7._

- - -

This enables API users to see whether a merge request can be merged without conflicts.

See merge request !2756

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