Installable as PWA - service worker & manifest

[joepio]

• start_url should match (currently does not, as the front-end fetches from github pages instead of /
• Icon images should resolve (currenlty uses relative paths which doesn't work)
• Add service worker. No matching service worker detected. You may need to reload the page, or check that the scope of the service worker for the current page encloses the scope and start URL from the manifest.
• Make sure the service worker caches the correct things

[joepio]

Current approach failed to work.

[joepio]

GoogleChrome/workbox#2978

[joepio]

Fixes didn't work.

[joepio]

Maybe I should use https://www.npmjs.com/package/snowpack-service-worker-assets

Not sure how / if it interoperates with workbox though