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)
Users that aren't members of an assign allowed group can no longer use the feature. Automatically unassign all their topics.
Rafael dos Santos Silva
Krzysztof Kotlarek
Jarek Radosz
Roman Rizzi