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/error_page.jsx

@@ -2,6 +2,7 @@
 // See License.txt for license information.
 
 import $ from 'jquery';
+import PropTypes from 'prop-types';
 import React from 'react';
 import {FormattedMessage} from 'react-intl';
 import {Link} from 'react-router/es6';
@@ -12,7 +13,7 @@ import * as Utils from 'utils/utils.jsx';
 
 export default class ErrorPage extends React.Component {
     static propTypes = {
-        location: React.PropTypes.object.isRequired
+        location: PropTypes.object.isRequired
     };
 
     constructor(props) {