Refactor class to fix todo
Refactor class to fix todo

I traded one RuboCop disable for another, haha! I'm actually pretty
happy with how this turned out - we now have only one call to
`PaperTrail#track_changes_with_transaction`, which I think is a big win
and I find this a little easier to read.

I did, however, decide to tell RuboCop to ignore line length inside of
`add_relationships` because it's either that or ignore the feedback that
I should use a guard clause. Other refactoring ideas I had just seemed
like a pain, so I bailed on them and just disabled the cop. Open to
other ideas, but I think this makes sense for now.

artsy/bearden,artsy/bearden,artsy/bearden,artsy/bearden