Use X-Forwarded-Host as url_root when present

[marekful]

If whoogle is behind a proxy which is accessed on a non-standard port, this port is lost to the application and element['src']s are incorrectly formed (omitting port). The port is also missing when redirecting after new session.

E.g. some unrelated service is running on example.com:80/443 and Nginx listens on example.com:5000 proxying to whoogle which listens on some host 5001. In this case, request.url_root doesn't include the front port 5000 which is expected by the client (browser) to render img.src correctly.

HTTP X-Forwarded-Host will contain this front port number in a typical Nginx reverse proxy configuratoin, e.g.

server {
    listen 5000;
    listen [::]:5000;
    server_name whoogle.example.com;
    ...
    location ~ /(search|) {
            proxy_pass http://10.0.8.14:5001;
            proxy_set_header Host $host;
            proxy_set_header X-Forwarded-Host $http_host; # will forward https://whoogle.example.com:5000/
            proxy_set_header X-Forwarded-Proto $scheme;
            proxy_redirect off;
    }
}