Fix Webpack manifest not working with single-threaded Web servers

The removal of the webpack-rails gem in !42485 (merged) changed the way the manifest was loaded from the Webpack dev server. Instead of contacting directly to the HTTP port 3809, the change caused an external HTTP(s) request to be made to the GDK URL (e.g. https://gdk.test:3001/assets/manifest.json). This would cause a single-threaded like Thin to lock up.

This change restores the separate manifest_host and manifest_port that was previously used to contact the Webpack server directly.