Display commit ID for discussions made on merge request commits

Closes #53953

See merge request gitlab-org/gitlab-ce!23837

Fix typos about .gitlab-ci.yml in Serverless docs

See merge request gitlab-org/gitlab-ce!23963

Fixes a bug where merge request comments made
in the context of a specific commit no longer specify
which commit they were created on

Display "commented" only for commit discussions on merge requests

Closes #53950 and #53681

See merge request gitlab-org/gitlab-ce!23622

Display "commented" only for commit discussions on merge requests (EE-port)

Closes gitlab-ce#53950 and gitlab-ce#53681

See merge request gitlab-org/gitlab-ee!8790

Use px instead of rems to prep for csslab

Closes #55314

See merge request gitlab-org/gitlab-ce!23804

Fix line-height of line numbers

Closes #55278 and #51668

See merge request gitlab-org/gitlab-ce!23775

Fix broken link in feature proposal template

See merge request gitlab-org/gitlab-ce!23957

Resolve "build-loader-animation dots misaligned"

Closes #52620

See merge request gitlab-org/gitlab-ce!23959

Add support for attaching epics to an epic

Closes #8333

See merge request gitlab-org/gitlab-ee!8637

Remove deprecated xhr from specs

Closes #55448

See merge request gitlab-org/gitlab-ce!23949

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Remove &:not(.use-csslab) from table to fix dark diff themes

Closes #55282

See merge request gitlab-org/gitlab-ce!23941

Removes batch_review_notification feature flag

See merge request gitlab-org/gitlab-ee!8933

Fixed merge spec conflict

Closes #8939

See merge request gitlab-org/gitlab-ee!8934

- we now use the hierarchy class also for epics
- also rename supports_nested_groups? into supports_nested_objects?
  - move it to a concern
- add checks for epics

Integrates frontend with API to enable
attaching epics to an epic

Add controller + services for:
 - setting a parent
 - list epics
 - delete associarion

Adds backstage support to related issues app
to enable attaching epics to an epic

Remove epics field for projects not in groups

Closes #7866

See merge request gitlab-org/gitlab-ee!8919

Add missing entries to locale/gitlab.pot

Closes #8940

See merge request gitlab-org/gitlab-ee!8935

Also fixes epics field being shown in MRs

CE port: Rename GroupHierarchy into ObjectHierarchy

See merge request gitlab-org/gitlab-ce!23923