Force the nick name to be unique with a DB constraint, with an issue
where the user name for the OLAT token is the auto generated identity
name after changing the password

REST add a user search by external id, remove the login value of UserVO
if identity name are automatically generated

Adapt LDAP for auto/manual generated id. name, add unit tests for
special workflow (user which are created without authentication and get
LDAP token afterwards on login)