Fix MR assignees for push options

Closes #60305

See merge request gitlab-org/gitlab-ce!27186

With multiple MR assignees, `merge_request.assignee_id` is always nil,
and `merge_request.assignee_ids` isn't in `merge_request.attributes`.
So the existing code doesn't set assignees in the created MR.

This fix gets all the tests passing, but we should also check that no
other associations in the MergeRequest need similar fixups.

Quarantine flaky test: "Discussion Comments Issue behaves like discussion comments clicking "Comment" will post a comment" / spec/support/features/discussion_comments_shared_example.rb:19

See merge request gitlab-org/gitlab-ce!27183

Extract preloaded_relations

See merge request gitlab-org/gitlab-ce!27168

Resolve "Extract EE specific files/lines for CI/CD app/serializers"

Closes #6649

See merge request gitlab-org/gitlab-ee!10670

Mark disabled pages domain for removal

See merge request gitlab-org/gitlab-ce!26212

Set pages_domain.remove_at when disabling it
Add specs for marking pages domain for removal
Notify user that domain is being removed
Add documentation

Ensure ID is big enough for prometheus_metrics records

Closes #11054

See merge request gitlab-org/gitlab-ee!10679

Fixed failing user views design spec

Closes #11056

See merge request gitlab-org/gitlab-ee!10682

Resolve "Extract EE specific files/lines for CI spec/serializers"

See merge request gitlab-org/gitlab-ee!10668

Quarantine flaky specs

See merge request gitlab-org/gitlab-ee!10680

Update comments about N + 1 Gitaly calls

See merge request gitlab-org/gitlab-ce!27178

Quarantine a flaky spec in spec/features/issues/user_interacts_with_awards_spec.rb

See merge request gitlab-org/gitlab-ce!27176

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/11056

Quarantine failing nightly tests

See merge request gitlab-org/gitlab-ee!10683

Fix SVG icon colors in Related MRs widget

Closes #60249

See merge request gitlab-org/gitlab-ce!27151

Fix icon colors of related MRs widget

Closes gitlab-ce#60249

See merge request gitlab-org/gitlab-ee!10660

Remove a "reopen merge request button" on a "merged" merge request

Closes #54039

See merge request gitlab-org/gitlab-ce!26965

Resolve "Make the `review-deploy` curl the Review App for a 200 response to decide success"

Closes #60229

See merge request gitlab-org/gitlab-ce!27134

CE port of Move "Authorize project access with external service" to Core

See merge request gitlab-org/gitlab-ce!27056

Move Contribution Analytics related spec in spec/features/groups/group_page_with_external_authorization_service_spec to EE

Move "Authorize project access with external service" to Core

See merge request gitlab-org/gitlab-ee!10437

...instead of EEExternalAuthorizationServiceHelpers

To make sure all known issues are linked to the correct epic, I've gone
through the code base, and updated the comments where required.

[Backport] Support multiple assignees for merge requests

See merge request gitlab-org/gitlab-ce!27089

Support multiple assignees for merge requests

Closes #2004

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

Fix grammar and try to make clearer how a fork works

See merge request gitlab-org/gitlab-ce!26929

add missing word in docs

See merge request gitlab-org/gitlab-ce!26882

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