Create Service Worker

[LukePrior]

Will allow site to load quicker and work offline with stored data when installed as a PWA.

Caching Strategy:

• Map tiles (keep for 30 days)
• historical NBN info (keep indefinitely)
• main/current NBN info (fetch every time or maybe keep for a few hrs max)
• JavaScript files and others (30 days?)

https://stackoverflow.com/questions/46036897/best-practices-for-detecting-offline-state-in-a-service-worker
https://developer.chrome.com/docs/workbox/caching-strategies-overview/

[LukePrior]

Got this for now: https://github.com/LukePrior/nbn-upgrade-map/blob/main/site/serviceworker.js

[LukePrior]

Ok hopefully that will work ok, still needs an update to periodically purge caches if nearing storage limits

[lyricnz]

This might help with #309

[LukePrior]

#309 (comment)