CE Backport: Epic issue list and related issue list re-design

See merge request gitlab-org/gitlab-ce!23658

Resolve "Navigating unresolved discussions on Merge Request page"

Closes #51122

See merge request gitlab-org/gitlab-ce!22789

Resolve "Extended user centric tooltips"

Closes #50157

See merge request gitlab-org/gitlab-ce!23231

[CE] Backport SAML unlink changes: UrlBlocker#ascii_only

See merge request gitlab-org/gitlab-ce!23627

Resolve "Utilize new charting library for metrics dashboard"

See merge request gitlab-org/gitlab-ce!23459

Integrate csslab

Closes csslab#1

See merge request gitlab-org/gitlab-ce!23521

Fix for incorrect Correlation-ID key being passed through GRPC metadata

Closes #55054

See merge request gitlab-org/gitlab-ce!23666

Add Kushal to frontend CODEOWNERS

See merge request gitlab-org/gitlab-ce!23664

Fill project_repositories for hashed storage projects

Closes #48527

See merge request gitlab-org/gitlab-ce!23482

Replace the metrics dashboard graph with
GitLab UI's area chart component

Disable docs lint internal_links check

Closes #55038

See merge request gitlab-org/gitlab-ce!23665

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

Closes #52007

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

This check never worked once the global navigation changes were rolled
out in https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/362. The
global nav will ALWAYS include /ce and /ee links, and the CE and EE
repos will never include documentation for both.  gitlab-docs now
expects the documentation to be in the following directories:

* /tmp/gitlab-docs/ce (CE)
* /tmp/gitlab-docs/ee (EE)
* /tmp/gitlab-docs/omnibus (Omnibus)

The problem is that the CE and EE repos will only include one of the
first two.

Let's disable this check until
https://gitlab.com/gitlab-com/gitlab-docs/issues/305 is resolved.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55038

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