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
Yeah, Agreed.,When I was developing this there are sometimes the revision was not created and the tests was raising NilMethodError rather than *a nice feedback about what happen*. I can remove this, but I prefer to move this to the spec, just to make sure that the revision is created.