Generate new React App for Admin Next

[kevinhughes27]

part of #750

This PR runs the create-react-app script to generate a new client along with all the build and development configuration. create-react-app is a project from Facebook to offer an abstracted and opinionated project setup for React applications. It hides away all the low level configuration and enables all the latest and greatest in Javascript development like hot reloading, small image in-lining, code splitting and more. It doesn't lock you in either since you can eject the abstracted configuration if you need to, however this will make updating to new versions harder.

For historic context create-react-app performs the same role as the Rails asset pipeline does. The main advantage is that it is actively maintained by frontend people instead of more or less being abandoned by backend people. Eventually we'll be able to remove the asset pipeline in this project and have separate toolchains for server and client projects.

create-react-app is nearing version 2 which includes a lot of great stuff but unfortunately its not ready yet. In the past they have provided very clear upgrade instructions. Since we're using typescript we'll also have to wait for the typescript fork to update.

• Roadmap for react-scripts@2.0 facebook/create-react-app#3815
• Migrate react-scripts to v2 wmonk/create-react-app-typescript#284

cc @patrickkenzie @JRSpencer @RichardBlair

[kevinhughes27]

btw I'm using VS Code as my editor for frontend projects now and Atom still for backend. VS Code has a lot of really nice things for Javascript and even more for Typescript