Forum: Per-discussion subscription

There are fairly extensive unit tests which accompany this issue, and behat tests which cover a selection of it.

The tests currently do not cover:

• moving discussions between forums; or
• backup/restore.

Moving discussions

1. create a forum with a subscription mode of Auto and a name of "Auto forum"
2. post a discussion in the forum with the subject "Automatic discussion"
   • Confirm that you are subscribed to the post
3. create a forum with a subscription mode of Optional and a name of "Optional forum"
4. post a discussion in the forum with the subject "Optional discussion"
   • Confirm that you are not subscribed to the post
5. View the "Optional discussion" discussion
6. Move the discussion to the Automatic forum
7. View the Automatic forum threadlist
   • Confirm that you are:
   • *subscribed to the "Automatic discussion"
   • *unsubscribed to the "Optional discussion"
8. View "Automatic discussion"
9. Move the discussion to the Optional forum
10. View the Optional forum threadlist
    • *Confirm that you are subscribed to "Automatic discussion"
11. View "Automatic discussion" again
12. Move the discussion back to the Automatic forum
13. View the Automatic forum threadlist
    • Confirm that you are:
    • *subscribed to the "Automatic discussion"
    • *unsubscribed to the "Optional discussion"
14. View the "Optional discussion" discussion
15. Move the discussion back to the Optional forum
16. View the Optional forum threadlist
    • *Confirm that you are unsubscribed from "Optional discussion"
17. Subscribe yourself to the "Optional discussion" discussion
18. View the Automatic forum threadlist
    • *Confirm that you are subscribed to "Automatic discussion"
19. Unsubscribe yourself from the "Automatic discussion" discussion
20. View the "Optional discussion" discussion (which you're now subscribed to)
21. Move the discussion to the Automatic forum
22. View the Automatic forum threadlist
    • Confirm that you are:
    • *unsubscribed from the "Automatic discussion"
    • *subscribed to the "Optional discussion"
23. View "Automatic discussion" (which you're no longer subscribed to)
24. Move the discussion to the Optional forum
25. View the Optional forum threadlist
    • *Confirm that you are now unsubscribed from "Automatic discussion"
26. View "Automatic discussion" again
27. Move the discussion back to the Automatic forum
28. View the Automatic forum threadlist
    • Confirm that you are:
    • *unsubscribed from the "Automatic discussion"
    • *subscribed to the "Optional discussion"

Backup/restore

1. Create a forum in optional subscription mode
2. Create two discussions within it
3. As student1:
   • Subscribe to discussion 1
4. As student2:
   • Subscribe to discussion 2
5. As student3:
   • Subscribe to the forum
6. As student4:
   • Subscribe to the forum
   • Unsubscribe from discussion 1
7. As student5:
   • Subscribe to the forum
   • Unsubscribe from discussion 2
8. As admin, backup the forum including user data
9. Restore the forum
10. For each of the users above: Confirm that the subscription data was copied over correctly

You should also move around the UI to check that it behaves as expected. Pay particular notice to:

• the discussion list - you should be able to toggle the discussion subscription from the discussions and have them update from one to other
• writing a new post - this displays information about your current subscription. It also defaults to subscribing you to the discussion when you post any message to that discussion