allow enrolment modules to modify the course edit form

The authentication plugin can lock fields on the user edit form, but the enrolment plugin can't modify the course settings form.

This is a problem, because our enrolment system pulls most of the data from an external system. We rely on certain fields being in a certain form to synchronize properly. Also, some fields will get overwritten when the synchronization is done. If we let teachers edit them, it causes confusion.

It should be fairly easy to have a method on the enrolment class that can modify the form.