Update the app whenever a new version is available

[gmarty]

Let's refresh the page whenever a new version is detected by the service worker.
To avoid disrupting the user, new updates should be applied during the night. If a new version is detected during day time, the update must be postponed to the following night.

Possible related topics: tag the app version in the code (e.g. a global VERSION constant) and log this value at startup in Google Analytics.