use webpack devServer / webpackHotDevClient in development mode

[christopher-johnson]

webpack has a devServer that works well for live reloading and hot module replacement. The dev server will always run on a different port than the e2e server so that testing can be done simultaneously with the dev environment. Optimally, a distinct webpack development configuration would accompany the implementation. The 'react-dev-utils/webpackHotDevClient' can be used for refreshing and hot updates.

See https://github.com/ubl-chj/research-and-demos/blob/history/packages/mirador3-app-base/config/webpack.config.dev.js and https://github.com/ubl-chj/research-and-demos/blob/history/packages/mirador3-app-base/scripts/start.js for reference.

closes #25