Add a Subscribable concern

[gravitystorm]

We should be able to refactor Changesets, Diary Entries (and potentially nodes) to share code as a concern - e.g. so that Changeset.subscribe(user), DiaryEntry.subscribe(user), Note.subscribe(user) are all implemented in the same manner.

An interesting topic here is whether we should have a polymorphic subscriptions model/table, rather than separate tables for each type of subscription. They will all just be object_id, user_id and nothing else, but if we wanted to change one (e.g. to add t.timestamps) then we would want to do the same to all of them. That suggests to me that a polymorphic table might be better, albeit hard to retrofit.