Add setting to control how long to keep emails before deleting them

[Robert Roskam]

Would require setting up celery tasks

[Jared Proffitt]

@raiderrobert what if we just delete any notifications that are older than the date setting whenever we send a notification? That way we wouldn't need a celery task.

[Robert Roskam]

That could be an interesting setting for each message. Basically it would act like a retention period.