OK, this is a new scheme to allow some teachers to edit and others to not

OK, this is a new scheme to allow some teachers to edit and others to not
that is much better than the proposal to change the role of course creators.

There is a new field in user_teachers called "editall", which is
ON BY DEFAULT, and allows teachers to edit courses.  It can be modified
on the teacher editing screen (formerly assign teachers).

The value is cached in the session.

To test for it, there is a new function isteacheredit($course->id)
which works much like isteacher did.

I'm going through now and applying this new function wherever
it is needed.