Upgrade to React 16

[jeremyyap]

To get to react 16, we need enzyme 3 (currently at 2 - migration guide).

• Upgrade to Enzyme 3

Enzyme does not currently support testing portals. Previously in enzyme 2 a workaround was used which was to obtain the ref to the portal element and wrap it in a ReactWrapper, but this no longer works in enzyme 3.
StackOverflow
enzymejs/enzyme#252
enzymejs/enzyme#1202

Maybe migrating to Material-UI v1 will resolve the portal issue? v0.x uses a custom portal implementation which eludes testing in Enzyme 3. In v1, ReactDOM.createPortal is used. Enzyme 3 seems to have support for React Portals.
mui/material-ui#9613
enzymejs/enzyme#1345

• Upgrade to React 16

References:
https://medium.com/@jameslockwood/getting-react-16-jest-and-enzyme-to-play-nicely-cc6d216ce3c4