Upgrade to Webpack 4 + Babel 7

[drewpc]

It's time, folks. This project is stuck on Webpack 1 and Babel 6 which limits a lot of what can be done, in addition to just being old. Webpack 4 and Babel 7 will poise us for the future and allow us to better integrate the Rollup changes proposed in #985 .

• Upgrade packages to use Webpack 4 and Babel 7
• Ensure sane default values for the react-server-cli generated Webpack configs
• Incorporate the use of webpack-dev-middleware instead of webpack-dev-server to simplify things (addresses proposal Consolidate HTML + Javascript Servers #807 and PR Consolidate HTML + JS Servers #808 (that was rejected)...gonna have to push this through due to unnecessary complexity here). This also removes the need for Add disableHostCheck to WebpackDevServer #932 and CLI still runs webpack build with --js-url=... #774

[drewpc]

Addressed in #1003