Updating client dependencies. Switching to yarn. (#6433)

* Updating client dependancies. Switching to using yarn.

* Updating React

* Moving pure components to using function syntax (performance gains with newer react version)

* Updating client dependancies.

* Ignore .yarninstall

* Enabling pre-lockfile because it's the entire point of using yarn.

* Removing old webpack config

* Moving to new prop-types

* Fixing ESLint Errors

* Updating jest snapshots.

* Cleaning up package.json

webapp/components/tutorial/tutorial_intro_screens.jsx

@@ -17,13 +17,15 @@ import AppIcons from 'images/appIcons.png';
 
 const NUM_SCREENS = 3;
 
+import PropTypes from 'prop-types';
+
 import React from 'react';
 
 export default class TutorialIntroScreens extends React.Component {
     static get propTypes() {
         return {
-            townSquare: React.PropTypes.object,
-            offTopic: React.PropTypes.object
+            townSquare: PropTypes.object,
+            offTopic: PropTypes.object
         };
     }
     constructor(props) {