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)
* Feature: Allow assign on groups
* Use type: group_list option
* Track group changes and update the setting accordingly. Restrict reminders frequency to assign allowed users instead of staff
Rafael dos Santos Silva
Krzysztof Kotlarek
Gerhard Schlager
Roman Rizzi
Guo Xiang Tan