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

common/djangoapps/course_modes/models.py

@@ -545,6 +545,19 @@ class CourseMode(models.Model):
         """
         return cls.PROFESSIONAL in modes_dict or cls.NO_ID_PROFESSIONAL_MODE in modes_dict
 
+    @classmethod
+    def contains_audit_mode(cls, modes_dict):
+        """
+        Check whether the modes_dict contains an audit mode.
+
+        Args:
+            modes_dict (dict): a dict of course modes
+
+        Returns:
+            bool: whether modes_dict contains an audit mode
+        """
+        return cls.AUDIT in modes_dict
+
     @classmethod
     def is_professional_mode(cls, course_mode_tuple):
         """