Implement offline mode

[kjk]

Currently without network connection Edna won't work even though it has all the files locally.

Implement offline support:

• https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Tutorials/js13kGames/Offline_Service_workers
• https://developers.google.com/codelabs/pwa-training/pwa03--going-offline#0
• https://www.monterail.com/blog/pwa-offline-dynamic-data
• https://medium.com/@louisjaphethkouassi/make-your-pwa-work-offline-with-a-service-worker-f95f16d93029