TaggableManager doesn't base itself off of RelatedManager

[rtpg]

TaggableManager is, essentially, a RelatedManager in spirit. It's fetching data off of a related table, has its prefetch cache.

Over the years we've had a couple bugs (#802 , #686 ) due to us not relying on RelatedManager and just reproducing features ad-hoc.

I haven't deeply looked into the feasibility of using RelatedManager directly. But if it's doable, it would be very good in my opinion. We might still need to do some magic to maintain some backwards compat.