Major Artist - Follow/Suggest - User relationship refactoring

Reasonably big changes so take note @Jellyfishboy:

Have removed several unnecessarily implemented methods that AR
implements for us (eg `user.follow_artist(artist)` replaced by
`user.followed_artists << artist`).

Relationships are now consistently named:
(artist | user).(follows | suggests) for the 'through' record.
(artist | user).(followed_x | suggested_x) for the 'belongs to' record.
E.g. radiohead.followed_users > all the users that follow radiohead.

I have refactored everywhere I could find the old uses, but our test
suite is weak so even though all tests are passing, there could be
unseen issues.