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 calendar web service to not use the identity name, add a new user
authentications web service based on identity key, remove the advanced
properties controller

Login happens only with authentication informations, identity name is
now only used for legacy directory name. User name of authentication can
be changed. LDAP users can be imported with LDAP::ldapUid. Users can be
imported in courses, groups... with First name last name.