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 think we should only remove the visibility restriction if the user is a moderator, or if we don't do that we should only render them in the view if the user is a moderator.

The class based client side show/hide stuff that you're using at the moment was only ever intended to be used where we couldn't (for caching reasons) do the restriction on the server, and that doesn't apply here (or anywhere really now as we have pretty much given up on caching).

On top of that, there may well be comments that we have to hide and which we really don't want showing up in the source at all even if the javascript does make them invisible.