[Feature] Can put rqbit Web UI behind proxy server

[ikatson]

This is related to features mentioned in #296

This allows you to put rqbit Web UI behind a proxy server under a different path, and it'll still work.

E.g. for nginx

        # Proxy requests to /rqbit/ to rqbit at http://localhost:3030
        location /rqbit/ {
            proxy_pass http://localhost:3030/;
            proxy_http_version 1.1;

            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        }

Then open URL/rqbit/web/ in your HTTP server

CC @mpmc