feat(frontend): improve project password handling

Never fetch the password in requests. Password is only mandatory on creation. Password field is left empty on update and only sent if filled.

Password is readable only by admins. Non-admin owners can still update the password like previously.

@famarger Je vois que la méthode virtuoso_credentials récupère self.virtuoso_password, est-ce que ce patch va casser son usage ?

Closes https://forge.extranet.logilab.fr/cubicweb/cubes/rodolf/-/issues/116