[core] Add http_x_forwarded_proto for x-forwarded-proto support

I've put an aws elb with https termination in front of hue.
Navigating to https://hue.mydomain/ returns a redirect to http://hue.mydomain/accounts/login/
Clearly it's picking up the domain correctly from the x-forwarded-for header, but it reverts to http, so it seems like it's ignoring the x-forwarded-proto header.

Is this an oversight? Is this mode of operation supported?

I expect I'll run into trouble with any urls that escape the elb i.e. if hue gives the user a link to something outside of hue.mydomain/