Remove paper trail

Our use of PaperTrail doesn't play nicely with Globalize3, which is what we're intending to use for i18n.

We were only using a small portion of PaperTrail which made it relatively easy to replace with a hand-rolled solution.

One important point to note is that we no longer store a serialized copy of the Edition in the versions table. We think this is OK because it was never being used. This Pull Request doesn't remove the historic serialized objects from that table but we'd advocate removing that soon.