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
Even though I get the point, this seems odd. A subscription does not get commented, a discussion does. Not sure of a good name for this, but I believe it requires further thought.