Replace Webpack extract-text-plugin with mini-css-extract-plugin

[pascalw]

Since Webpack 4 extract-text-plugin is deprecated for usage with css, instead https://github.com/webpack-contrib/mini-css-extract-plugin is recommended. Supposedly it's faster too.

See https://github.com/webpack-contrib/extract-text-webpack-plugin#usage.

[ruisalgado]

yes I have this change pending review on the project I'm working on... I'll contribute the change to maji