Finish support for excluding teachers from student enrollments
 - teacher enrollments are collected across all sections of a course
 - after all sections of a course are processed, look to see if any
   added enrollments were in the list of teachers (this can happen if
   a student enrollment was processed for a section that came before
   another section where the same user was enrolled as a teacher)
 - if any are found, remove them from the enrollment CSV file before
   submitting

To test:
 - in a test course synced with a section that's sync'd from a maillist,
   add kipling as a teacher to a section that comes after the sync'd one
 - delete kipling's sync'd student enrollment
 - run the enrollment script for those sections and ensure Kipling does not
   get added back