Add isAuthorized()

Overrides AppController::isAuthorized(), which only allowed admins
access to restricted actions
Change "student's" to "your"
Fix referring to nonexistant named parameters in isAuthorized()
Add comment
Remove references to nonexistent
/course_registrations/take_off_waiting_list
Update isAuthorized()
Add missing loadModel()
Allow access to route

Since authorization is determined in the action
Remove reference to nonexistent action
Remove isAuthorized()

Since there aren't any denied actions here
Allow admins to remove students from classes
Add order ID to refund email
Add user_name and email to JWTs
Refactor JWT generation
Edit purchase_id validation to allow for either numbers or null
Remove association with Course
Remove $course_id parameter from Release::isSubmitted()
Change Release::getIdFromUidCid($user_id, $course_id) to
getIdFromUserId($user_id)
Add conditional success message
Add validation for Release.birthdate