This changes some of the checks to use the existing `author_association`
where possible, which will result in no change of effect - this is also
what GitHub uses internally.

The advatage is that it saves us a few requests, especially in the
context of the GitHub worker, and allows us to share code more
efficiently here. It also removes some ugly duplication I wanted to get
rid of for some time and makes the code overall more readable IMO.

Release Notes:

- N/A