Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

This new endpoint is intended to replace enrollment API call used on the login+registration page. Instead of directly enrolling students, the view will contact the external e-commerce API (Oscar) to create a new order. Oscar will be responsible for completing the order and enrolling the student.

This behavior will only apply to course modes with associated SKUs. All other course mode enrollments will be processed directly by LMS.