Automate service worker versioning

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Bump the service worker version number in the python template here as part of the build or deploy process.

Whenever JavaScript changes are made to the app, the service worker version should be bumped before redeploy, or else the app may behave unexpectedly due to outdated, cached files. Bump the service worker version at least as frequently as with every deployment build to prevent such issues without requiring a manual version bump. Once deployed, the service worker version should remain stable, so it can retain valid cached files.