Add multiple MR assignees migration and background table population

What does this MR do?

1. Add migration required for multiple assignees
2. Add post-deployment migration which schedules batches background migrations to insert merge_request_assignees rows
3. Add a after_save callback to keep merge_request_assignees in sync while the migrations doesn't finishes and the codebase is still not using the relation

EE-port https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10411

Query generated on background migration

INSERT INTO merge_request_assignees (merge_request_id, user_id)
SELECT "merge_requests"."id",
       "merge_requests"."assignee_id"
FROM "merge_requests"
WHERE (NOT EXISTS
    (SELECT 1
     FROM merge_request_assignees
     WHERE (merge_request_assignees.merge_request_id = merge_requests.id) )
  AND ("merge_requests"."id" BETWEEN 1 AND 4)
  AND (assignee_id IS NOT NULL)

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/2004

Does this MR meet the acceptance criteria?

- [ ] Changelog entry added, if necessary

• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer