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
Allowing markdown would be lovely, but we have the same problem as with notes - trying to render anything more sophisticated than text in the space available in the sidebar is kind of problematic.