HTTP[S] Proxy Support

Up until now, we have supported IXP Manager behind a proxy with some
custom variables. However, Laravel added built in support for this
in 5.5.

This commit removes our custom code in favour of the official version.

See: http://docs.ixpmanager.org/install/runtime/#behind-a-https-proxy

NB: Please make sure you have correctly set APP_URL in your local .env file.

NB 2: If upgrading from <4.7.0, please upgrade to v4.7.0 first.