Be more explicit when adding restricted users lists
https://framacolibri.org/t/tests-urgent-besoin-de-retours-rapides-sur-la-rc-de-mypads/419/3
from bayartb : When adding new user, we should use email adress instead of (or in alternative of) existing logins. If the user email adress is corresponding to an existing user, this user must be added. If not, an email should be sent and the pad administrator warned that email(s) were being sent.