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
Maybe `new_comment`. It's not ideal, but at least it does not imply that the subscription is commented, just that here is a new comment for that subscription.