Before, notes (discussions.json) would be fetched twice in the following
case:
* User has a "Show comments only" or "Show history only" filter
* User then goes to an issue with a note hash, i.e. #note_*
* Frontend then makes two notes requests:
  1. Notes for the user's original filter
  2. Notes for "Show all activity" so that the note is definitely shown

This commit fixes this race condition so that there is always only
one note request made whether it is for a specific note or not.