1 - allow me to allocate a room change to a covered lesson even if I haven't yet allocated a teacher (I often get advance notice of room changes needed but can't do anything unless I allocate a 'fake teacher' first)
2 - leave the room change as I booked it even if I then change the cover teacher (currently you have to change the teacher, then click 'cover options' again to secure the room change, otherwise it drops off when you click 'save' on the new teacher choice)
