[BPK-971] Upgrade webpack deps

[Matt Davidson-Loubet (matthewdavidson)]

This is one of the many PR's aimed at upgrading dependencies in the backpack monorepo. I focus on updating webpack based dependencies. Here are the list of changelogs:

• https://github.com/postcss/autoprefixer/blob/master/CHANGELOG.md
• https://github.com/webpack-contrib/extract-text-webpack-plugin/blob/master/CHANGELOG.md
• https://github.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md
• https://github.com/postcss/postcss-loader/blob/master/CHANGELOG.md
• https://github.com/webpack-contrib/style-loader/blob/master/CHANGELOG.md
• https://github.com/webpack/webpack/releases
• https://github.com/webpack/webpack-dev-server/releases

Also added --open flag to automatically open web browser for docs dev.

[Backpack Bot (backpackbot)]

	Warnings
⚠️	One or more packages have changed, but changelog.md wasn't updated.

	Messages
📖	Thanks for the PR 🎉.

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[Matteo Hertel (matteo-hertel)]

Awesome!! 🎉

[Matt Davidson-Loubet (matthewdavidson)]

I've tested storybook dev (npm start), docs dev (npm run docs) and docs prod output (npm run docs:dist && http-server dist).

[Hugo Tunius (k0nserv)]

LGTM.

Surprised we didn't have to make any code changes with the Webpack 3 upgrade etc

[Matteo Hertel (matteo-hertel)]

LGTM

[Shaun Donnelly (shaundon)]

I couldn't find in the links you posted where this change comes from - what does it do?

[Matt Davidson-Loubet (matthewdavidson)]

extract-text-webpack-plugin added sorting https://github.com/webpack-contrib/extract-text-webpack-plugin/blob/master/CHANGELOG.md#300-rc1-2017-07-07

[Matt Davidson-Loubet (matthewdavidson)]

Hugo Tunius (@k0nserv) me too - for comparison, here is CRA's webpack 3 upgrade PR facebook/create-react-app#2574