We are already checking that assignee has access to the private message. However, admin still can be assigned as technically they have access.
We should ensure that assignee has direct access to the message.
If `SiteSetting.unassign_on_close` is enabled, an Assignment record can still exist, but be set to inactive, when a Topic is closed.
Inactivement Assignments should be ignored when comparing to the maximum number of assigned topics.
When topic is closed, we mark assignments as "inactive". Then when it is reopen and setting reassign_on_open is enabled, we bring back previous assignments.
We were already using custom fields for archive message and move to inbox. I changed custom fields solution to use active flag on Assignment model
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)
Krzysztof Kotlarek
Natalie Tay
Jarek Radosz
jbrw