This removes some complexity from the event_queue module.
To avoid code duplication, we reduce the `is_notifiable` methods to
internally just call the `trigger` methods and check their return value.
We will later consistently use these functions to check for notifiable
messages in the message send and event_queue code.
We have these functions accept the `sender_id` so that we can avoid the
`private_message = message["type"] == "private" and user_id != sender_id`
wizardy.
