included more tests for TeamMaker and users/merge

Some of the new tests include error checking.
added tutors field to courses add/edit

Previously tutors can only be added to courses via the API or
users add/edit (by admins). I have now added a tutors field
to courses add/edit that is simiar to the instructors field.
This will allow instructors to enrol/unenrol tutors from their
course.

On the side I have also fixed users/edit for tutors. The list of
courses the tutors have access to are not checked in users/edit.
This has now been fixed.
included more system tests

I've added new tests for enrolling students, importing groups, and
importing users.