Make react theme production ready

[thangngoc89]

Currently, the react theme (created with bsb -init my-app -theme react) is not production ready. And people complains about hello world app size.

I added some improvements to the react theme and it's now on bar with CRA hello world bundle (~118 KB before gzip)

What I did:

• react 16
• webpack 4 with production mode
• es6 module (for tree-shaking bs-platform's stdlibs)

The result is here https://github.com/thangngoc89/reason-react-boilerplate and I would love for this to be merged upstream