Added the PUT operation in CourseController to edit courses by a user who is an admin/instructor AND is apart of the staff list of the given course. Created tests for a staff editing a course, a non-staff can't edit a course, and a staff cannot edit a non-existent course. Also added a custom exception for the user not being in the staff list of the given course.
Added the PUT operation in CourseController to edit courses by a user who is an admin/instructor AND is apart of the staff list of the given course. Created tests for a staff editing a course, a non-staff can't edit a course, and a staff cannot edit a non-existent course. Also added a custom exception for the user not being in the staff list of the given course.
Closes #27