Add a service-worker to enable caching and desktop PWA #138

On slow connections code-server takes ages to load the main JS bundle file, as it is multiple megabytes large. When reloading the page the bundle file has to get loaded again unless the browser locally caches it, which is not certain. It would drastically improve the experience if a service-worker would cache all of the necessary html, css, and JS bundles.

In combination with a web-manifest this would also enable code-server to be installed on client devices as PWA's. This is especially useful for desktop as you would not have to install a thin client. Using a PWA would also enable code-server to eventually implement features such as share-target to upload files onto the remote container.

This would require a very minimal amount of work for a very very useful feature, especially when you use Workbox.