[desktop] Fix HTTPS termination using X-Forwarded-Proto header

When Hue is behind a load balancer that is terminating HTTPS it needs to rely on a special header (sent by the load balancer itself) to send proper urls.

The fix is using the header X-Forwarded-Proto (see https://docs.djangoproject.com/en/1.9/ref/settings/#secure-proxy-ssl-header ) instead of a custom one (X-Forwarded-Protocol).