Changes to allow much better control over what "teachers" are called

Changes to allow much better control over what "teachers" are called
in a course.  Firstly, the course settings page now allows the teachers
to specify the word they want to use in place of "teachers" and "students"
as well as "teacher" and "student".  Secondly, a new teacher admin tool
allows any teacher to modify the order and displayed role of teachers
in that course.  This affects the display on the course listings, the
participants page and so on.