Error with mini-css-extract-plugin

[einbandi]

There was an error in a recent version of mini-css-extract-plugin, see this issue.

I resolved it temporarily by pinning the version of the mini-css-extract-plugin.

The error was resolved in a new release today, but Netlify still uses the broken version. We shouldn't forget to remove "mini-css-extract-plugin": "2.4.5" at some point.