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
I had this called `updated` initially, but that could cause confusion with rails methods, and we might have other events other than comments which will update the discussion but might trigger different actions

But I'm open for better suggestions