Optimise our CSS output

[tomdye]

We should consider using https://github.com/NMFR/optimize-css-assets-webpack-plugin to optimise our css output. This is especially important if we enable css-variables because otherwise there is a lot of duplication in the output css (because most widget css files import variables.css).

The main issue previously is that using this plugin breaks our sourcemaps. We need to find the correct / appropriate properties to get this working correctly.

Enables: #62