Add PWA offline support for standalone web app install

[stevenpovlitz]

Addresses #182

This is my first PR to meshtastic and meshtastic/web, please point out contribution policies and conventions. The code is pulled from the default service worker setup from running:

npx create-react-app my-app --template cra-template-pwa-typescript

[CLAassistant]

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[CLAassistant]

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