Feature/update section classlist logic

This change set fixes the logic used to retrieve the list of enrollments that are eligible to be added to the currently selected section.  The view function starts with retrieving all enrollments tied to the given course, and then calls a newly separated utility function to filter out undesired enrollments.  The resulting set should be all of the unique user and role combinations that make up the enrollment list, minus any that are contained in the current section (while also filtering out the test student).