Generate relative urls in server.js

[last-g]

This change will allow using server behind a reverse proxy.

[welcome]

🙌 Thanks for opening this pull request! You're awesome.

[Fenny]

This is exactly what I need

[alxhotel]

Tested with the following configuration in NGINX and webtorrent server listening at port 37717:

location /proxy/ {
	proxy_pass http://localhost:37717/;
}

Works as expected.

[feross]

Thanks for the PR @last-g. Thanks for reviewing and testing it @alxhotel.

The code in this file changed, so there's now a merge conflict. I just manually made this change and released 0.107.10. ✨