Add paper trail

@danmaclean This PR adds PaperTrail as discussed. I've decided against the Paper Trail Manager gem for now until we decide what we're tracking with PaperTrail and if it's really necessary.

On Monday, we should chat regarding what you want to track with this gem. It's powerful enough to track everything but I'm wary of the larger datasets that you have and storing superfluous information so we need to nail down _exactly_ what you want to audit.