Add command to delete old notifications

[rubickcz]

Hoarding of old notifications is bad for several reasons, e.g.:

• database gets bigger -> filtering gets slower
• it is hard to make data migrations with high notifications count

Notifications are usefull at time they arrive, but like 3 months later, they are pretty useless. The goal is to implement a Django command, that will delete notifications older than e.g. 6 months (this should be configurable via settings)

NOTE: the command should also delete templates that have no notifications associated with them.