There is an active flag for assignments. It is used to bring assignments back when topic is reopened.
However, when assignment is inactive, it should not be displayed on assigned list or search.
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)
Adds three new search modifiers:
- in:assigned for assigned topics
- in:unassigned for unassigned topics
- assigned:{username} to list topics assigned to a specific user
These modifiers are all made available in the advanced search sidebar
Krzysztof Kotlarek
Jarek Radosz
Ahmed Gagan