Fix N+1 queries for comment authors in the notes API

Fix N+1 queries for comment authors in the notes API

Preload the authors of note comments, and of the notes themselves for
the RSS feeds, when rendering notes from the API.

The tests now use notes with comments by at least three different
users, as prosopite doesn't always detect an N+1 query with only two
records - the first query in a process can have a different call stack
while the association's statement cache is built.