Adds subscriptions for each discussion

When a discussion is created, all users are subscribed to it by default

The state of the subscription can be:
* new: someone else created the discussion
* unwatched: Old discussion that I'm not following
* read: No new updates
* unread: New updates on a discussion I'm following

Whenever a comment occurs, every one who is not the comment's author and is subscribed to the discussion gets his state changed
Yeah... That would not make sense, but would read better. Either way, maybe `first_unread_comment` would better explain what it is.