Limit tries sending email before cancellation

[atymic]

I had a bug where one of the notifications was updated, but the previously serialized version had some invalid data in it.

This caused problems because snooze tries (and fails) to send it every minute.
We should have some kind of 'try' limit, to make sure we don't keep trying (and failing) to send the same notification.

[atymic]

cc @thomasjohnkane

Guess we'd need a migration chance, to add some type of tries column