Allow site to run properly behind an Nginx proxy

I have an Nginx server setup such that https://example.com/starbound works for CommandStar with the following rules:
```
location /starbound/ {
    rewrite ^/starbound/(.*) /$1 break;
    proxy_pass http://127.0.0.1:8080;
    include proxy.conf;
}
```
Since the CommandStar HTML was referencing absolute paths ("/css/", etc.), my rewrite was failing to pull in any additional assets or load the API, etc. With this change, everything works with the proxy remap to /starbound/.