Fix and improve ServiceWorker configuration
Because of a misconfiguration, the service worker currently always prefetches the JS files for ES2015- and ES5-compatible browsers. The config should be adjusted in a way that only the files for ES2015 are prefetched. Futhermore, the config should be improved to prefetch important WOFF2 webfonts which are already used on the home page (Material icons, 400 and 500 font weight versions of Roboto).
These changes would improve performance for current browsers at the expense of performance for legacy browser which do not support ES2015 or WOFF2.