Try out mini-css-extract-plugin's new HMR support

[edmorley]

mini-css-extract-plugin v0.6.0 added initial support for HMR (which would mean we could drop use of style-loader), however there have been a few bugs reported, so we did not enable it as part of #1353.

However we should take a look once the feature matures.

See:
webpack-contrib/mini-css-extract-plugin#334
https://github.com/webpack-contrib/mini-css-extract-plugin#hot-module-reloading-hmr

[edmorley]

It looks like there are still quite a few bugs with mini-css-extract-plugin's HMR support, so perhaps not worth switching to it quite yet:
https://github.com/webpack-contrib/mini-css-extract-plugin/issues?q=is%3Aissue+is%3Aopen+hmr