* 9-4-stable: (41 commits)
  Also inject new route helpers into includers of GitlabRoutingHelper
  Remove many N+1 queries with merge requests API
  Change `sign_out` usage to `gitlab_sign_out` in one spec
  Simplify stage_id migration as we now use relations
  Do not schedule bg migration when it is not needed
  Schedule stage_id bg migrations in batches of 10
  Use new `each_batch` helper instead of custom one
  Revert recent changes in migration helpers
  Add some comments on new migrations helpers
  Schedule stage_id background migration in ranges
  Extract `execute_in_batches` migrations helper
  Add walk_table_in_batches and refactor migration helpers
  Reduce a delay between stage_id scheduled migrations
  Improve exception description in bg migrations
  Do not override original AR5 batching interface
  Sanitize id value passed to async background migration
  Improve code examples in background migrations docs
  Add description to exception in bg migrations worker
  Do not compare float with integer in migrations specs
  Improve readability of build stage id migration query
  ...

Conflicts:
	spec/features/boards/boards_spec.rb
	spec/features/issues/create_issue_for_single_discussion_in_merge_request_spec.rb
	spec/features/issues/user_uses_slash_commands_spec.rb
	spec/features/merge_requests/edit_mr_spec.rb
	spec/features/milestones/show_spec.rb
	spec/features/protected_tags_spec.rb