Add system hooks for merge requests

Closes #4519

See merge request gitlab-org/gitlab-ee!3924

CE upstream - Thursday

See merge request gitlab-org/gitlab-ee!4139

Add the :migration metadata, and don't use factories in FixWronglyRenamedRoutes spec

See merge request gitlab-org/gitlab-ce!16535

Resolve "Cannot filter issues by labels ending with a plus char in board view"

Closes #41715 and #34252

See merge request gitlab-org/gitlab-ce!16490

CE upstream - Thursday

See merge request gitlab-org/gitlab-ee!4132

Remove unused CSS selectors for Cycle Analytics (between 1st iteration and 2nd iteration)

Closes #41743

See merge request gitlab-org/gitlab-ce!16270

CE upstream - Wednesday

See merge request gitlab-org/gitlab-ee!4128

Double padding for file-content wiki class on larger screens

See merge request gitlab-org/gitlab-ce!16534

Filter groups and projects dropdowns of search page on backend

Closes #38739

See merge request gitlab-org/gitlab-ce!16336

Signed-off-by: Rémy Coutable <remy@rymai.me>

https://gitlab.com/gitlab-org/gitlab-ce/issues/34252

[ci skip]

Fix bug in security release with deploy keys migration

See merge request gitlab-org/gitlab-ce!16531

# Conflicts:
#	app/controllers/projects/deploy_keys_controller.rb

[ci skip]

Further simplify app/views/projects/buttons/_dropdown.html.haml

See merge request gitlab-org/gitlab-ce!16503

CE upstream - Wednesday

Closes #4378

See merge request gitlab-org/gitlab-ee!4126

Backport 10.3.4 security fixes into master

See merge request gitlab-org/gitlab-ce!16509

[EE] Port 10.3.4 security fixes into master

See merge request gitlab-org/gitlab-ee!4122

Signed-off-by: Rémy Coutable <remy@rymai.me>

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16506
See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4112

From https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4126

# Conflicts:
#	Gemfile.lock
#	app/assets/javascripts/dispatcher.js
#	changelogs/unreleased-ee/4378-fix-cluster-js-not-running-on-update-page.yml
#	spec/requests/api/jobs_spec.rb
#	spec/requests/api/v3/builds_spec.rb

[ci skip]

CE upstream - Tuesday

See merge request gitlab-org/gitlab-ee!4106

Open visibility help in new tab

Closes #39591

See merge request gitlab-org/gitlab-ce!16268

Docs: document new redirect for Disqus comments

See merge request gitlab-org/gitlab-ee!3976

Resolve "Failure in spec/migrations/fix_wrongly_renamed_routes_spec.rb"

Closes #4619 and #4622

See merge request gitlab-org/gitlab-ee!4119