Activity log

Activitiy log feature. These PR are dependent from this PR on comfortable mexican sofa: https://github.com/moneyadviceservice/comfortable-mexican-sofa/pull/11
I would push this into `revision` again. Perhaps `revision.type` that would return `:note` or `:event` depending on the type. There is lots of feature envy going on in here. This class knows so much about the structure of `revision`.