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/pdf_preview.jsx

@@ -5,6 +5,8 @@ import FileInfoPreview from './file_info_preview.jsx';
 
 import loadingGif from 'images/load.gif';
 
+import PropTypes from 'prop-types';
+
 import React from 'react';
 import PDFJS from 'pdfjs-dist';
 import {FormattedMessage} from 'react-intl';
@@ -171,6 +173,6 @@ export default class PDFPreview extends React.Component {
 }
 
 PDFPreview.propTypes = {
-    fileInfo: React.PropTypes.object.isRequired,
-    fileUrl: React.PropTypes.string.isRequired
+    fileInfo: PropTypes.object.isRequired,
+    fileUrl: PropTypes.string.isRequired
 };