Incompatibility between mini-css-extract-plugin 0.4.3 and the manifest plugin

[Lyrkan]

While working on #403 I noticed that the build was failing when upgrading all of our dependencies to their latest allowed version.

It seems that this is caused by an incompatibility between mini-css-extract-plugin@0.4.3 and webpack-manifest-plugin: webpack-contrib/mini-css-extract-plugin#177 (comment)

I opened an issue on the Webpack Manifest Plugin repository (shellscape/webpack-manifest-plugin#167), but meanwhile maybe we should force mini-css-extract-plugin to use v0.4.2?

[Lyrkan]

Duplicate of #415