Create a ChangesetSubscription model

[gravitystorm]

We have various subscription models, such as NoteSubscription and DiaryEntrySubscription. We also have a table "changeset_subscribers" which does the same thing (pairs of user_id/object_id), but we don't have a corresponding model. Instead we declare it as a join association in Changesets, and use custom methods on the changeset model (e.g. changeset.subscribe(user)) to indirectly add and remove the rows in that table.

I think it would be worth refactoring this code, to make subscriptions behave the same way across all three models. So we should

• Create a ChangesetSubscription model, although it will need self.table_name = "changeset_subscribers"
• Refactor changeset_subscriptions_controller to use the ChangesetSubscription model, likely through changeset.subscriptions.new
• Remove the subscribe/unsubscribe methods from the Changeset model, since these aren't acting on the Changeset model itself.