The merge request widget has a section that includes which issues may be
closed or mentioned based on the merge request description. The problem
is that rendering and redacting Markdown can be expensive, especially
since the browser polls for the data every 10 seconds.

Since these links don't change much and are just nice to have, we only
load them on first page load. The frontend will use the existing data if
the data doesn't appear on subsequent requests.

This saves about 30% of the rendering time of this endpoint, which adds
up to significant savings considering that
`MergeRequestsController#show.json` is called over a million times a day
on GitLab.com.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63546

This reverts merge request !26853

Use serialization for project boards

See merge request gitlab-org/gitlab-ce!29263

With this commit, we add docs link for confidential
and locked issues to note compose and edit forms

- Add serializers and Grape::Entity
- Replace to_json
- Add specs

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

* Add Pipeline IID to MR's
* Add Pipeline IID to commit box
* Add Pipeline IID to builds page
* Add Pipeline IID to job sidebar
* Add Pipeline IID to header on view pipeline pages
* Add changelog entry
* Update tests

Extracts combining the job_artifacts relation with the with_reports
scope for getting report artifacts into a method.