ProgramCourseEnrollments were already unique on
(program_enrollment, course_enrollment) by nature of the
OneToOneField on course_enrollment. However, this only
affects realized enrollments. For waiting enrollments, we
need to add a uniqueness constraint on
(program_enrollment, course_key).
This makes the LMS program_enrollments API compatible with the
Registrar programs API, as specified in the Master's partner
integration guide.
EDUCATOR-4389
