Fix pseudo n+1 queries with Note and Note Authors in issuables APIs

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.

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