Closes #18484

Based on the feedback from reviewers.

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

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

This was not a clear cut n+1 query, given that if you're directly subscribed to all issues
that the API is returning you never really need to check for the notes. However, if you're
subscribed to _all_ of them, then for each issuable you need to go once to `notes`, and
once to `users` (for the authors). By preemtively loading notes and authors, at worst you have
1 extra query, and at best you saved 2n extra queries. We also took advantage of this preloading
of notes when counting user notes.

* commit 'ca3c5c295ed653b483fe81c3918ffe60f46666b9':
  Let contributors know where to start
  Ensure branch cleanup regardless of whether the import process succeeds
  Fix failing todo tests
  Reorder the todos because the use of the project finder attempts to order them differently
  Update target todo test to use a public project
  Use the project finder in the todos finder to limit todos to just ones within projects you have access to.
  Move filtering todos by projects not pending deletion into a scope on the todo model
  Reduce the filters on the todos joins project query by being explicit about the join
  Ensure we don't show TODOS for projects pending delete
  Fix deprecation warnings in spec/services/issues/bulk_update_service_spec.rb
  Remove unused Issuable#is_assigned? method
  fixup! Don't allow merges with new commits
  fixup! Add `sha` parameter to MR accept API
  Reduce Namespace queries in UserReferenceFilter
  Added ReferenceFilter#nodes
  Returning enums in ReferenceFilter#each_node
  Don't allow merges with new commits
  Add `sha` parameter to MR accept API

The `sha` parameter is optional, and when present, must match the
current HEAD SHA of the source branch. Otherwise, the API call fails
with a 409 Conflict and a message containing the current HEAD for the
source branch.

Also tidy up some doc wording.

Closes #15638