Add `NGINX_X_ACCEL_REDIRECT` setting. Check this instead of `DEBUG` when...

... deciding to use the `X-Accel-Redirect` header.

The current behaviour is overloading the `DEBUG` setting for multiple purposes. By de-coupling them, I can deploy to non-Nginx environments and I can also independently change `DEBUG` during development for enhanced error reporting without altering the actual behaviour of the application.