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
Yes, this whole feature was renamed to "Changeset Discussions" because of
the comment confusion (see my talk at SOTM EU around this), but a
discussion contains multiple "comments"- though I guess we could change the
term to "post" or something that's less generic.

- Serge