Add support for subscribing to an individual thread

On some very busy mailing lists, it's not worthwhile to subscribe to the entire list. It would be nice to be able to subscribe to a single thread in the archives and be notified when new posts arrive. Ideally it would be possible to receive the complete message to your configured email address and reply to it, but even just a notification that a new message has appeared in the archive so that the user can go to the web UI view to read and possibly reply would be fine.