(cherry picked from commit 01470b39)

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

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.

Implement an upgrader which delete unused course /export/* directory or
download ZIPs like imscp.zip, blog.zip, podcast.zip which not unused or
only temporary