Bump React to version 16.14.0

[rhystmills]

Co-authored-by: @jonathonherbert and @samanthagottlieb

This PR bumps react in tagmanager to 16.14.0, which will allow us to make use of React packages using hooks (introduced in 16.8.0) - for example the upcoming prosemirror-editor.

Bumping to 17+ will be more complicated - some of our dependencies are still using the deprecated componentWillMount, componentWillReceiveProps and componentWillUpdate methods - which are removed in React 17 - leaving only their UNSAFE_... equivalents.

A few things that came up in the 15 > 16 React bump (may be relevant to atom-workshop and targeting if/when we do it there):

• The componentWill… and componentDid… lifecycle methods are now deprecated. To continue to use past React 16, use UNSAFE_component… instead. They're not 'unsafe' in terms of security, but apparently encouraged bad development practices: https://legacy.reactjs.org/blog/2018/03/27/update-on-async-rendering.html
  -prop-types comes from its own library now. Import from there instead of using React.PropTypes
• the moment localiser has been moved to a separate package from version 3 > 4 (necessary to be compatible with React v16).
• react-transition-group needed a bump - meaning that import CSSTransitionGroup from 'react-transition-group/CSSTransitionGroup' became import {CSSTransitionGroup} from 'react-transition-group';

I've fixed a couple of pre-existing missing key prop warnings as well.

I did attempt bumping react-router but that one looks complicated as the API has changed - browser-history will come from a separate package, Link comes from a separate package (react-router-dom), IndexRoute no longer exists, and the app now needs to be wrapped in a Router or BrowserRouter among other snags.

How to test

Run tagmanager locally according to the instructions in the readme, or deploy to CODE. Does the app run as expected? Do you see any React-themed errors?
(There will be some warnings about deprecated methods, but no errors)

[dblatcher]

LGTM