Incorrect auth and socket urls when Centrifugo works on custom location

[FZambia]

I.e. when we use sth like this in Nginx:

server {
     listen 30620;

     location /centrifugo/ {
               rewrite /centrifugo(.*) $1 break;
               proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
               proxy_set_header Host $http_host;
               proxy_redirect off;
               proxy_pass http://127.0.0.1:30619;
       }
}

We must probably use relative auth path and use location path to generate socket url.