Enhance versioning/updating

[Otto-AA]

Current Behavior

Currently the app is cached via a service worker, so the files only get loaded once and then remain the same. Loading a new version must be done manually (e.g. by a complete refresh). Also there's no way to find out the currently used version.

Expected behavior

The service worker should check for updates and display a notification if one is available. The user can then decided to update the app or stay with the old version. There should be an info menu in which the user can view the version number and potentially update the app.