Framework: Bump Webpack dependency versions

[aduth]

This pull request seeks to bump Webpack-related dependencies to their latest versions.

Relevant changelogs for versions introducing breaking changes:

• https://github.com/webpack/webpack/releases/tag/v3.0.0
• https://github.com/webpack-contrib/extract-text-webpack-plugin/releases/tag/v3.0.0
• https://github.com/postcss/postcss-loader/releases/tag/v2.0.0

Testing instructions:

Verify that npm run dev and npm run build complete with no errors.

Ensure that PostCSS configuration is still applied as expected, specifically in applying prefixes automatically. An example is to confirm that -webpack and -ms prefixes are applied to box-align properties on .editor-saved-state.

Ensure there are otherwise no regressions in the editor display.

[youknowriad]

Works great 👍

[aduth]

Seems to be working happily enough, but I should have bumped the babel-loader dependency as well. It's currently warning on npm install.