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 comment, as now it doesn't only mean checking that the username and password are correct, but can mean verifying the LDAP login is valid. Suggested change:
"Check that the username/password combination or LDAP credentials are valid"