Make websockets work when behind a proxy

This makes webmpc work correctly when behind an (https) proxy.
It fixes the detection whether or not `wss://` should be used and makes the websocket path used relative to the current directory.

example nginx config for this:
```
location ^~ /music {
    rewrite                /music/(.*) /$1 break;
    rewrite                ^/music$ /music/ permanent;
    proxy_pass             http://localhost:8080/;  
    proxy_redirect         / /music/;
    proxy_set_header       Host $host;
    proxy_buffering off;

    proxy_set_header Upgrade     $http_upgrade;
    proxy_set_header Connection  "upgrade";
}
```