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
* FEATURE: Remind users of assigned tasks.
* User an user custom field instead of adding a new column
* Improve tests. fix assigns count and display oldest assigned topics correctly. Do not remind about recently assigned tasks