Update to webpack 4

[Scarysize]

This is currently automatically installed:

    "webpack": "^4.12.0",
    "webpack-dev-server": "^3.1.4"

We need to add:

    "webpack-cli": "^3.0.8",

And replace extract-text-webpack-plugin with mini-css-extract-plugin. The updated webpack config should look like this: https://github.com/ubilabs/turner-wayfinder/blob/master/webpack.config.js#L32