To host at mydomain.com/getslack, can't use absolute urls. Also,
socket.io needs to be initialized with the proper path.

Sample nginx config to host in a subfolder:

```
location /getslack/ {
    proxy_pass http://localhost:3000/;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    # upgrade needed for websocket connection
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
}
```