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/admin_console/sync_now_button.jsx

@@ -1,3 +1,5 @@
+import PropTypes from 'prop-types';
+
 // Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
@@ -12,7 +14,7 @@ import {ldapSyncNow} from 'actions/admin_actions.jsx';
 export default class SyncNowButton extends React.Component {
     static get propTypes() {
         return {
-            disabled: React.PropTypes.bool
+            disabled: PropTypes.bool
         };
     }
     constructor(props) {