This commit adds a service worker to make the app installable as a PWA.
In addition to caching all generated build artifacts (css and js filse)
the service worker also caches all other static files via a
stale-while-revalidate strategy (png, ico, json, css from the public
folder).
Further, this commit adds a Notification which will be shown when the
service worker updates and clicking on the button inside it causes
the service worker to update and the page to reload.
In order to make a PWA installable we need to follow some guide lines:
https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Guides/Making_PWAs_installableFixes: #147
