We will use this later to display which user group was mentioned
in push and email notifications.
`mentioned_user_group_ids` is kept as a List (not Set) to ensure proper
test coverage of the function, since it depends on the order of iteration,
and we cannot change the order of iteration for a set (which we'll need
to do for proper testing).
Part of #13080.
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.
Abhijeet Prasad Bodas