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 understanding is you want the more selective first in composite indexes, which probably means a unique index on ``[:changeset_id, :subscriber_id]`` and ``[:subscriber_id]`` for the other.