Add service worker support for a nicer offline experience

[holandes22]

This is low priority, but it can really enhance the UX if connection goes down. We can show previously fetched content instead of an ugly disconnected error in case the connection goes down.

Todo:

• Verify bundled chromium in electron supports service worker
• Service worker requires the server to have SSL enabled, so until that is done this cannot be active (although service worker should fallback gracefully to just show the disconnected error)
• UpUp js lib can really help out here

[stale]

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