This change makes it possible to associate an assign notification with the `assignments` record that creates the notification. It'll be needed for the assignments tab in the new experimental user menu, see https://github.com/discourse/discourse/pull/17379.
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)
Ability to assign groups.
To assign group, user must have a right to send a message to that group.
In addition, 2 jobs were introduced, - AssignNotification and UnassignNotification to inform interested users in the background about the new assignment.
* 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
Osama Sayegh
Rafael dos Santos Silva
Natalie Tay
Krzysztof Kotlarek
Jarek Radosz
Guo Xiang Tan
Roman Rizzi