would be nice to have a notification class in action operations - "user[s] who acknowledged this"

it should send a message to those who have added ack to the event, originally triggering the notification. probably should look in all sequential problem events, even if there are unknown events inbetween.
should be careful with users that might have been deleted since the acknowledge was added.

if this is added, maybe also worth adding the opposite target, users who haven't added ack message yet