Webpack 5 migration prep (update css loaders and webpack config)

[VIKTORVAV99]

This PR is the start of a long process to get us ready to upgrade to webpack 5.

It:

• updates all the css loaders to the highest version that still support webpack 4.
• switches the use of autoprefixer to postcss-env (it includes autoprefixer among other plugins)
• removes the custom function to output mainfest.json as it's not supported by webpack 5
• adds the BundleAnalyzerPlugin to output the manifest.json file again
• clean up the webpack-config.js file by removing options that won't be supported in webpack 5 or changing them to supported versions.

[VIKTORVAV99]

Turns out there won't be a long process!

I have a stable version that use Webpack 5 locally and can push it here or add a separate PR after this is one merged.

Let me know what you think is best!

[madsnedergaard]

Lets do a separate PR to make testing easier :)
I'll review this today!

[madsnedergaard]

Tested and AFAIK it works just fine - so let's try it! 🙃

[VIKTORVAV99]

Hehe dev dependencies are the "safe" ones.

It's when we need to update the production dependencies I start to worry. 😅