Show a notification when there is an app update

[gregjhogan]

We use a service worker to update the app in the background when a new version is released.

The notification should communicate there is an update and what you have to do to load it (you need to close the entire browser, all tabs, to get the update to load)

Simply refreshing the page could also work if we are comfortable using skipWaiting()
https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle