Update Assignment Sharing

Share a devoir as an objet commun (colleagues or whole school), update its sharing, or make it private again (empty values). Any collaborator with write access can manage the sharing, not only the owner ("un truc très partagé"). Unlike the historical share-with-teacher endpoint this does not copy anything: everyone edits the same devoir until a class somewhere starts working on it, which freezes it (see services/assignment_sharing.py).