Changeset discussion feature

New ChangesetComment model and join table between users and changsets for comments subscriptions
Made commeting changesets possible via "browse" interface
Comments feed for all changsets, particular changeset
Email notifications of new comments on "subscribed to" changesets
My logic was that when we fetch subscribers for a changeset we won't care about the order, but when we fetch changesets for a subscriber we might - if for example we provided a paged list of changesets you were subscribed to.