Invalidate service-worker-cached app shell and content

[dillonkearns]

#52 makes sure that the app shell is cached. But if there are changes, then ideally we should tell the service worker whenever we find out it is stale.

We can have the content.json file include a hash that tells use whether the service worker cache is up-to-date with what we're fetching in content.json.

[dillonkearns]

Not relevant anymore since the service worker isn't running right now. We can revisit service workers in the future but the whole thing will be looked at from scratch.