Rewrite Notification subsystem

Rewrite Notification subsystem

In this commit:
 - delete a lot of really bad and / or unused code
 - redesign a much simpler Notification model
 - create and send notifications to the user's inbox on booking start & end
 - migrations
 - emails user when booking is ready and when it ends

Not in this commit:
 - Creating notifications from lab messages
 - warning messages when a booking is about to end
 - creating "summary" notifications when e.g. a booking has been fulfilled by a lab

Change-Id: I69b4dc36c3f2bce76d810106baadeef5a562cc7d
Signed-off-by: Parker Berberian <pberberian@iol.unh.edu>