Change `topic_id` to polymorphic approach (In the next step we will allow assigning to individual post)
`topic_id` column is still used for efficient display of assigned users on topic list (to avoid scanning posts)
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.
Krzysztof Kotlarek
Jarek Radosz
Guo Xiang Tan
Robin Ward