Display custom hook error messages when automatic merge is enabled

Closes gitlab-ce#29125

See merge request !1670

Port of fix/29125-ee to CE

Closes #29125

See merge request !10796

Services=>Integrations

See merge request !1655

CE upstream

Closes #2175, gitlab-ce#30810, gitlab-ce#27655, and gitlab-ce#30863

See merge request !1664

Do not append issuable state to links with custom anchor

Closes #30916

See merge request !10782

Refactor into .vue files part 2

See merge request !10791

Fix PlantUML integration in GFM

Closes #30557

See merge request !10651

Highlight Contribution Analytics tab under groups when active, remove sub-nav items

Closes #2204

See merge request !1677

Use constants in Gitlab::MarkupHelper

See merge request !10741

Refactor changing files in web UI

See merge request !1268

Refactor changing files in web UI

Closes #28711

See merge request !9323

Add retries to system hook worker

Closes #31109

See merge request !10801

Don't delete a branch involved in an open merge request in "Delete all merged branches" service

Closes #29427

See merge request !10727

The default number of Sidekiq retries is 25 over about 21 days.
If an external system is unavailable for a length of time, hooks
can pile up in the Sidekiq queues. If the situation gets bad enough
the retry jobs dropping back into the main queue can slow down
other jobs. Limit the retries to a sane number to avoid this
scenario.

Resolve "start discussion toggle clicking divider causes UI break"

Closes #30951

See merge request !10715

Add unique index to system note metadata

See merge request !10774

Port of 30985-cancel-pipelines to EE

See merge request !1676

Implement search by extern_uid in Users API

See merge request !10509

Add renderNote animation and added spec

Closes #29977

See merge request !10623

Set the issuable sidebar to remain collapsed on mobile devices

Closes #28086

See merge request !10323

Fixed branches sort toggle being empty

Closes #30950

See merge request !10764