Handle Multiple File Uploads in a service worker

[gavindoughtie]

For our exploratory code we're just encrypting/uploading a single file, but really should queue up multiple files, allow the user to attach metadata that our contracts will operate with, and perform the actual uploading resumably via a service worker.

References:

https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers

https://developers.google.com/web/updates/2015/12/background-sync?hl=en

https://github.com/clytras/pwa-sandbox

https://stackoverflow.com/questions/45709654/handling-file-uploads-when-offline-with-service-worker