Specify user privilege via LDAP or DBI

I added a method, UserPrivileged, to the ExternalAuth system.

In LDAP it works like the normal group check, except if the user is in the group specified, they are granted Privilege.

In DBI it works like the UserDisabled check by looking up a value in the configured field and comparing it to the list of values given in RT_SiteConfig.

I am using the LDAP version in production right now and it is stable. The DBI portion is entirely untested.