DEICH-6325 - Upgrade to WebPack 5 + cache busting 🚀

• Upgraded to webpack 5 🎉

  • Fixed module reload for WebPack 5, however could only make it work with sockjs.
  • Added automatic cleanup for public folder - otherwise it gets full with different bundle cache busted bundle versions.

• Added cache busting 🎉

  • The webpack config was modified to add contenthash to the generated bundle names (both js, and css). HtmlWebpackPlugin is used to generate public/index.html based on template stored in ./assets/index.html.
  • global.css was moved inside the src/css folder, and now included in App.svelte. All declarations were converted to have global scope. This was required to avoid caching global.css, thus invalidating cache busting partially.

Closes DEICH-6325