[Cashtab] Have refreshes update version for PWA users on update

Summary:
Cashtab has existing logic to push update notifications to users. This has been robust on the webapp but run into some issues with the PWA experience.

Digging into it, it's a complex issue.

Following this documented approach -- add code that helps make sure a refresh actually does pull in the new version.

https://redfin.engineering/how-to-fix-the-refresh-button-when-using-service-workers-a8e27af6df68

Difficult to test this since we need to know how it impacts cashtab.com PWA users. In local testing, it does seem to push out updates reliably.

Test Plan:
What I did
npm run build
netlify deploy --prod --dir=build
...pushes this live
then change a component, e.g. the "Send to" form label on the Send screen to "Send to has changed"
npm run build
netlify deploy --prod --dir=build
Refresh page in PWA, see udpate modal. On update, the form label has changed.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15212