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
I'd like @zerebubuth to comment on the actual API changes - he's back now but I don't know if he missed my previous pokes while he was away...