Enhance the Link Program Enrollments Support Tool to handle the case of...

Enhance the Link Program Enrollments Support Tool to handle the case of linking a learner to a ProgramEnrollment that is already linked to a different edX account.

In this case,
* unenroll the currently linked learner from the courses associated with related ProgramCourseEnrollments
* for each course the is enrolled in as part of a ProgramCourseEnrollment, if the coures has an audit track, then move the enrollment into the audit track. Otherwise, keep the learner's enrollment in the existing track.
* link the new user to the ProgramEnrollment
* enroll the new user in the Master's track in the courses associated with related ProgramCourseEnrollments