This change makes it possible to associate an assign notification with the `assignments` record that creates the notification. It'll be needed for the assignments tab in the new experimental user menu, see https://github.com/discourse/discourse/pull/17379.
Ability to assign groups.
To assign group, user must have a right to send a message to that group.
In addition, 2 jobs were introduced, - AssignNotification and UnassignNotification to inform interested users in the background about the new assignment.
Osama Sayegh
Rafael dos Santos Silva
Natalie Tay
Krzysztof Kotlarek
Martin Brennan
Bianca Nenciu
Jarek Radosz