sso: Remote login blocked by our apache config

Our apache config for plinth restricts access to local IPs: https://github.com/freedombox/Plinth/blob/master/data/etc/apache2/sites-available/plinth.conf

This not only prevents you from accessing Plinth remotely, it also blocks applications that use sso like syncthing.

I think we at least need a way to turn off the IP restriction.