Add event to email notifications

[TimWolla]

Events should be added to the email notification system to support different external services using a plugin (e.g. instant messengers). Plugins should be able to cancel the outgoing email to completely replace emails with the external service.

• [ ] Add event to UserNotificationHandler::sendInstantMailNotification()
• [ ] Add event to DailyMailNotificationCronjob (and possibly move the functionality in there into UserNotificationHandler).
• [ ] Ensure there is a cancelEmail parameter.

[TimWolla]

It probably is most useful to look into this after #4000 (especially since cancelling is desired).