LDAP auth

Adds simple LDAP authentification. Uses additional config:
auth_backend: use "LDAP" to active LDAP authentification
ldaphost: LDAP server hostname
ldapport: LDAP server port
ldaprdntpl: user RDN template (%login% replaced by actual login), example: uid=%login%,ou=people,dc=example,dc=org

Please update this line again, as now it doesn't only mean that the username and password are correct, but can mean that the LDAP login is valid. Suggested change:
"Username/password combination is valid, or LDAP authentication was succesful"