Normalize destination data (save storage space, improve performance)

There's some destination data (uid, method, destination) which is carried accross all the notifications tables. The plan is to create a 'messaging_destination' table that will generate a unique id for each (method, destination).

Advantages:
- Save storage space and speed up notifications queries (specially the queue related ones)
- Allow notifications tracking for anonymous users, the 'notifications_sent' table (now we won't use an uid for anything)
- Provide unsubscribe links for anonymous users. As we have a unique key for each destination we can remove in one hit all the subscriptions for that destination.