Serve Javascript bundle from CDN

[ben-pr-p]

As part of #1233, we should build and serve our Javascript bundle from a CDN.

To do this, we need to:

• Add a step to all GitHub action's that pushes ./bundle.<hash>.js to a Cloud CDN routed Google Cloud Storage bucket whenever a release occurs. I believe that's only here. It looks like this action will help with that.
• Set up routing so that the bundle is discoverable at cdn.rewired.dev (or cdn.spokerewired.com?) @bchrobot ?
• Initially, we can leave the built bundle also available in the Docker container so we can easily switch between serving via CDN and serving via NodeJS via env.STATIC_ASSET_PATH. Once we have a few releases under our belt, we can remove that.

[bchrobot]

If we stick with the manifest map approach from webpack-manifest-plugin then we will need to copy the generated assets.json to the Docker image.

bundle.js is the only asset generated by the webpack build, however, so it may be easier (at least for production) to scrap the manifest approach in favor of a BUNDLE_URL envvar.