Update `sideEffects` property for webpack 4 (#402)
Update `sideEffects` property for webpack 4 (#402)

`Calendar.css` is not bundled in production mode because `sideEffects` is false.
https://stackoverflow.com/questions/49160752/what-does-webpack-4-expect-from-a-package-with-sideeffects-false
wojtekmaj/react-calendar,wojtekmaj/react-calendar,wojtekmaj/react-calendar