CSS isn't minified when building for production

[asbjornh]

It seems like support for minification using css-loader was dropped in 1.0.0 (changelog).
The scaffolded project uses ^1.0.0 and still uses the minimize option. I suspect we might have a few sites in production with un-minified css 😄 (I can confirm at least one).

The solution is also described in the changelog

[pellebjerkestrand]

I'll start work on this.