Update config to be compatible with Webpack 4

[scott1702]

The JS seems to build fine but the CSS fails as extract-text-webpack-plugin is deprecated for Webpack 4.

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

⚠️ Since webpack v4 the extract-text-webpack-plugin should not be used for css. Use mini-css-extract-plugin instead.

[maxime-rainville]

We're still on webpack 2 and I don't think we have any urgency to upgrade to webpack 4 any time soon.

Is this holding you back in some way?

[scott1702]

It's not blocking, it just means I'm restricted to using webpack 2 in modules where I want to take advantage of silverstripe/webpack-config.

[maxime-rainville]

I'm not sure it would be possible to develop a CMS module that uses a webpack version different from the silverstripe/admin one.

We're not committed to any specific timeframe in terms of webpack upgrade, but last time we had a discussion about it, our thinking was that it might be a SilverStripe 5 thing.

[GuySartorelli]

We're doing this as part of CMS 5. Closing in favour of silverstripe/silverstripe-admin#1312