fix assigning default role 'Student' to user without deleting enrollments on course delete
STUD-1140
update tests
STUD-1140
add tests for users 'Student' role and course_enrollments
STUD-1140
don't unseed_permissions_roles when a course is deleted and assign student role to all previously enrolled users
STUD-1140
code update/cleanup
STUD-1140