When topic is closed, we mark assignments as "inactive". Then when it is reopen and setting reassign_on_open is enabled, we bring back previous assignments.
We were already using custom fields for archive message and move to inbox. I changed custom fields solution to use active flag on Assignment model
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.