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
We need to add indexes here on both `subscriber_id` and `changeset_id` or things will quickly collapse as we build up subscribers ;-) What I would suggest is a unique index on `[:subscriber_id, :changeset_id]` which will also make duplicate entries impossible, and an ordinary index on `[:changeset_id]` for finding the subscribers to a changeset.