Feature user-management autoenroll

This is a recreation of PR 1906

Remove single enrollment/unenrollment
Make unenrollment work with multi-email listbox.
Provide checkbox for auto-enroll on activate if user not yet signed up.
Enroll student when activating if auto-enroll flag set.

**** This PR has a db migration (added student_courseenrollmentallowed.auto_enroll)