This task is based on discussion on MDL-32125.

MDL-32125 will fix the auto subscription mode only. This one will try to fix the "force subscribe" behavior.

The "force subscribe" method do not actually manually subscribe the users. It only consider all user of the course as subscribed but no one is inserted in the subscribe list. So, when you switch from force subscribe mode to another one, it seams that a unsubscribe occurs but its not. It only return to its original manually subscribed users.

Maybe we should subscribe manually all users when we select the force subscribe mode so when we switch to another mode, the users will all remain subscribed.

To reproduce:

1. Create a forum with Optional subscription
2. Check the subscribers (should be none)
3. In "Forum administration", click on "Edit settings"
4. Change the subscription mode to "forced subscription", then click on "Save and display"
5. Check the subscribers (should be all there)
6. Change the subscription mode to "Optional subscription", then click on "Save and display"
7. Check the subscribers (is now none, may be all there)