Expose `RAILS_ENV` as `process.env.NODE_ENV`
Expose `RAILS_ENV` as `process.env.NODE_ENV`

As seen in [optimizing performance][1], React makes use of the
`process.env.NODE_ENV` to conditionally exclude code when creating a
production build.

This pull request uses exposes the value of RAILS_ENV inside webpack, so
the production build can be smaller.

[1]: https://github.com/facebook/react/blob/30c7c4fbe6f35af93ddec0c7ae661d592821b296/docs/docs/optimizing-performance.md

nickpith/webpacker,usertesting/webpacker,nickpith/webpacker,usertesting/webpacker,rossta/webpacker,Davidzhu001/webpacker,Davidzhu001/webpacker,gauravtiwari/webpacker,gauravtiwari/webpacker,rossta/webpacker,shakacode/webpacker_lite,rossta/webpacker,rails/webpacker,rails/webpacker,rossta/webpacker,gauravtiwari/webpacker,usertesting/webpacker,Davidzhu001/webpacker,nickpith/webpacker