WIP: Sanitize DNs before storing in extern_uid
requested to merge dblessing/gitlab-ee:group-sync-dn-matching-may-not-be-as-smart-as-it-could-be-365 into master
Closes #365 (closed)
Clean up DNs before storing in extern_uid and before comparing them for group sync. Also add a migration that doesn't depend on any models.
cc/ @jacobvosmaer