Before this commit, the reminder was leaking the total assigned topics count in the title.
In the post raw it was ensured that it was not being leaked but the title was still leaking the count.
Also add specs and updated existing specs to ensure that the count is not leaked in the title.
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