[React] Add PWA features

[samuelmeuli]

This PR adds PWA features to the React app. All PWA-related Lighthouse checks will pass with these changes.

PWA-related changes:

• Improved the web app manifest with theme colors and icon links
• Added favicons in different sizes
• Added a custom service-worker setup. The default one generated by create-react-app is not sufficient because it only covers files which are part of the React app, i.e. not AEM resources that are requested from other paths

I've tried to add as many code comments as possible, so the implementation details should hopefully be self-explanatory. @godanny86, please let me know if I can clarify anything :)

Other required changes:

• Switched from node-sass to sass (Dart implementation) for compatibility with the latest macOS version
• Updated react-scripts (create-react-app) and aem-clientlib-generator because the newer versions include changes that allow code splitting and better SPA asset handling with AEM ClientLibs
• Updated the aem-clientlib-generator config to the latest one from the archetype (same reason)

[samuelmeuli]

Yes, that's a good idea. I've added the banner component to both PRs :)

[samuelmeuli]

@godanny86 As discussed, I've replaced the ServiceWorkerHeaderFilter Sling filter with a custom Dispatcher config (copied from WKND Sites and extended by a rule which adds the Service-Worker-Allowed header to the service-worker.js file, see a42cadd).