Changed frontmatter filtering to support YAML, JSON, TOML, and arbitrary languages

Closes #52007

See merge request gitlab-org/gitlab-ce!23331

Resolve "Hide variables in UI by default"

Closes #20422

See merge request gitlab-org/gitlab-ce!23518

[CE] - Add milestones autocomplete for epics

See merge request gitlab-org/gitlab-ce!23660

Ability to override Issuer Email for Cert Manager

See merge request gitlab-org/gitlab-ce!23503

Allow public forks to be deduplicated

See merge request gitlab-org/gitlab-ce!23508

When a project is forked, the new repository used to be a deep copy of everything
stored on disk by leveraging `git clone`. This works well, and makes isolation
between repository easy. However, the clone is at the start 100% the same as the
origin repository. And in the case of the objects in the object directory, this
is almost always going to be a lot of duplication.

Object Pools are a way to create a third repository that essentially only exists
for its 'objects' subdirectory. This third repository's object directory will be
set as alternate location for objects. This means that in the case an object is
missing in the local repository, git will look in another location. This other
location is the object pool repository.

When Git performs garbage collection, it's smart enough to check the
alternate location. When objects are duplicated, it will allow git to
throw one copy away. This copy is on the local repository, where to pool
remains as is.

These pools have an origin location, which for now will always be a
repository that itself is not a fork. When the root of a fork network is
forked by a user, the fork still clones the full repository. Async, the
pool repository will be created.

Either one of these processes can be done earlier than the other. To
handle this race condition, the Join ObjectPool operation is
idempotent. Given its idempotent, we can schedule it twice, with the
same effect.

To accommodate the holding of state two migrations have been added.
1. Added a state column to the pool_repositories column. This column is
managed by the state machine, allowing for hooks on transitions.
2. pool_repositories now has a source_project_id. This column in
convenient to have for multiple reasons: it has a unique index allowing
the database to handle race conditions when creating a new record. Also,
it's nice to know who the host is. As that's a short link to the fork
networks root.

Object pools are only available for public project, which use hashed
storage and when forking from the root of the fork network. (That is,
the project being forked from itself isn't a fork)

In this commit message I use both ObjectPool and Pool repositories,
which are alike, but different from each other. ObjectPool refers to
whatever is on the disk stored and managed by Gitaly. PoolRepository is
the record in the database.

Reenable CODEOWNERS

See merge request gitlab-org/gitlab-ce!23381

Docs: Fix wrong example url (`repositories` instead of `repository`)

See merge request gitlab-org/gitlab-ce!23377

Backports changes made to One notification per code review

See merge request gitlab-org/gitlab-ce!23656

CE backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8632

Resolve "Further improvements to Project overview UI"

Closes #51243

See merge request gitlab-org/gitlab-ce!22196

Merge branch '20422-hide-ui-variables-by-default' of https://gitlab.com/gitlab-org/gitlab-ce into 20422-hide-ui-variables-by-default

CE Port of "Web Terminal FE"

See merge request gitlab-org/gitlab-ce!23626

Remove RemoveOldWebHookLogsWorker

Closes #52592

See merge request gitlab-org/gitlab-ce!23628

Log and pass correlation-id between Unicorn, Sidekiq and Gitaly

See merge request gitlab-org/gitlab-ce!22844

Update merge request's merge_commit after branch update

Closes #48889

See merge request gitlab-org/gitlab-ce!22794

- Adds coverage for hide/reveal toggle button behavior

Improve error message when attempting to reopen MR and there's a new open MR for the same branch

Closes #22548

See merge request gitlab-org/gitlab-ce!22326

Backports some changes from gitlab-ee!7885

See merge request gitlab-org/gitlab-ce!23637

Fix transaction pollution in Shard.by_name

See merge request gitlab-org/gitlab-ce!23655

The EE merge request can be found here:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8442

Documentation for "`only/except: merge-requests` for merge request pipelines"

Closes #54511 and #15310

See merge request gitlab-org/gitlab-ce!23397

Continues the iteration on the project overview UI:

- moved star, fork and new clone button
(copy SSH/HTTPS URLs) to top right,
  made them smaller
- avatar is now larger (64px)
- 'Request access' is now a link instead
  of a button
- overview comes before the description +
  changed styling and added icons
- description font-size is now 16px (large-paragraph)
- quick links to files are moved downwards
  below the commit/pipeline info
- margins changed to group content into
  4 groups to clean up the interface
- visibility info reduced to icon-only and
  moved to the right of the title

Show primary button when all labels are prioritized

Closes #54467

See merge request gitlab-org/gitlab-ce!23648

[Documentation] Fix minor issues in search API doc

Closes #54959

See merge request gitlab-org/gitlab-ce!23645