Allow teaching staff to switch student -> staff

[pwnall]

We've seen a fair amount of accidents where staff register as students for their class. It's unlikely that some training or UI change will make this sort of user error go away, so Igor must provide a way for existing course staff to correct this error.

[pwnall]

This hasn't been implemented yet because we don't staff to be able to delete a student's registration by switching student -> staff and then revoking the staff role. This concern can be addressed by implementing an ops log that records all the things where we wipe data. This list currently includes the ability to remove a submission after the deadline, which is necessary as a way to correct accidentally submitting a document with sensitive info.