Notifications are now always recorded

Email notifications records are now marked as sent (along with a sent
time) instead of being destroyed.

This allows us to see how many notifications we are sending per time
period, and detect bugs in the notification code more accurately.

Tests included to ensure behaving as expected, all tests I have in this
spec pass.

@Jellyfishboy I did this in master in the hope to deploy it
immediately, however there are too many tests failing for me to risk
it, so I will simply merge it into staging. Commenting you as you may
be interested to know I've done this!