Support LDAP Authentication

[kopfi]

To allow configurations where the user can login with a single set of credentials across multiple applications, it would be useful to support LDAP-based authentication.

Although this behavior can already be achieved through the "external authentication" option, an configurable LDAP binding would make the process more direct.

The required parts for the authentication are already provided by Spring Security. I think, what remains to be done, is adpting the login-configuration to reflect the new options and configure the LdapAuthenticationProvider accordingly.