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