Support multiple assignees for merge requests

Closes #2004

See merge request gitlab-org/gitlab-ee!10161

Expose pipeline variables via API

Closes #28105

See merge request gitlab-org/gitlab-ce!26501

Used to get the variables via the API endpoint
`/projects/:id/pipelines/:pipeline_id/variables`
Signed-off-by: Agustin Henze <tin@redhat.com>

Enable ESLint rule jest/no-identical-title

See merge request gitlab-org/gitlab-ce!27139

Enable ESLint rule jest/no-identical-title (EE-port)

See merge request gitlab-org/gitlab-ee!10651

Remove unused CSS class in Related MRs widget

Closes gitlab-ee#10962

See merge request gitlab-org/gitlab-ce!27150

Remove unused CSS class in Related MRs widget

Closes #10962

See merge request gitlab-org/gitlab-ee!10659

Always show instance configuration link

Closes #59730

See merge request gitlab-org/gitlab-ce!26783

Stable reviewer roulette

See merge request gitlab-org/gitlab-ee!10640

Stable reviewer roulette

Closes #57766

See merge request gitlab-org/gitlab-ce!27117

docs public key -> private key

See merge request gitlab-org/gitlab-ce!26902

Automatically enable multiple MR assignees feature flag

See merge request gitlab-org/gitlab-ee!10558

Extract due quick action to shared example

Closes #59570

See merge request gitlab-org/gitlab-ce!26918

Quarantine flaky specs

See merge request gitlab-org/gitlab-ce!27170

Git push options to create a merge request, set target_branch and set merge when pipeline succeeds

Closes #53198 and #43263

See merge request gitlab-org/gitlab-ce!26752

Fix backport of project_creation_level migration

See merge request gitlab-org/gitlab-ce!27167

EE: Fix backport of project_creation_level migration

See merge request gitlab-org/gitlab-ee!10672

Fixes stylelint warnings in environments

Closes #59903

See merge request gitlab-org/gitlab-ce!27131

Related to:

- https://gitlab.com/gitlab-org/gitlab-ce/issues/60270
- https://gitlab.com/gitlab-org/gitlab-ce/issues/60271
- https://gitlab.com/gitlab-org/gitlab-ce/issues/60272
- https://gitlab.com/gitlab-org/gitlab-ce/issues/60273Signed-off-by: Rémy Coutable <remy@rymai.me>

Extract EE specific files/lines for spec/features/merge_request

Closes #6574

See merge request gitlab-org/gitlab-ee!10612

Extracts EE-specific Merge Request feature specs to /ee

This schedules a background migration that checks whether
`multiple_merge_request_assignees` feature flag needs to
be enabled. The rule for it is that
every PopulateMergeRequestAssigneesTable background job
cannot be still running, or being retried or dead.

If there are any enqueued jobs in the state above, the
"checker" (MergeRequestAssigneesMigrationProgressCheck) will
be rescheduled for 3 hours ahead. If there are only dead jobs
in the queue (i.e. no scheduled/enqueued or retrying jobs), we
raise an error and stop the rescheduling.

This commit provides the required code for making
creation and update of MRs with the `assignee_id`
params backward compatible.

Additionally, it provides the required interface
for the MergeRequest model to make old and new
exported GitLab versions handle the `assignee_id`
well.

This mainly handles the search and replace approach
to using assignees instead assignee for merge requests.

It reuses existing logic from Issuable, where now
issues _and_ merge requests handles multiple assignees.

The feature is behind a feature flag, though, there's no
way to switch back and forth to using the old and new tables.
That's because we'd need a "sync" approach: Once the feature
is off, we'd have to sync old table with new table, once it's
on, new table with old table. Using the new
`merge_request_assignees` altogether will make things simpler to
handle, even though it's a bit "riskier".

Docs: Add examples for linking to header IDs

Closes #57191

See merge request gitlab-org/gitlab-ce!26091

Docs: Fix anchors related to issues

See merge request gitlab-org/gitlab-ce!27165

Use US English for content

See merge request gitlab-org/gitlab-ce!27154