WL-171 when enrolling a student from a PaidCourseRegistration or a Registration...

...Code redemption, then note the CourseEnrollment id in the respective tables
The cart could potentially contain items other than PaidCourseRegistration, so the course_enrollment field would be missing from the item. This function will be removed from the PR https://github.com/edx/edx-platform/pull/6196, but still pointing out here.