Reply email notifications

This PR resurrects our notifier.py and greatly simplifies it, only handling reply-notifications for now.

The changes kill any connection to the user subscriptions table, (If everybody agrees, we can also remove that from our model), removes the customsearch template which was to be introduced for subscribing to streams (we can resurrect this later).

Of course many parts of this will be refactored later when we'll design our next-level notification system.