WIP: lets try to add service accounts
I did a first try on adding service accounts but as discussed already in #infra, the LDAP Models can't deal with inheritance as needed.
The main difference for service accs is the UID-range, I therefore added two new config variables
LDAP_USER_MAX_SERVICE_UID. On creation of a new user we need to chose an unused UID from this range, depending on the new checkbox