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/admin_sidebar_section.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.
 
@@ -9,14 +11,14 @@ import * as Utils from 'utils/utils.jsx';
 export default class AdminSidebarSection extends React.Component {
     static get propTypes() {
         return {
-            name: React.PropTypes.string.isRequired,
-            title: React.PropTypes.node.isRequired,
-            type: React.PropTypes.string,
-            parentLink: React.PropTypes.string,
-            subsection: React.PropTypes.bool,
-            children: React.PropTypes.arrayOf(React.PropTypes.element),
-            action: React.PropTypes.node,
-            onlyActiveOnIndex: React.PropTypes.bool
+            name: PropTypes.string.isRequired,
+            title: PropTypes.node.isRequired,
+            type: PropTypes.string,
+            parentLink: PropTypes.string,
+            subsection: PropTypes.bool,
+            children: PropTypes.arrayOf(PropTypes.element),
+            action: PropTypes.node,
+            onlyActiveOnIndex: PropTypes.bool
         };
     }