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/sidebar_header.jsx

@@ -1,3 +1,5 @@
+import PropTypes from 'prop-types';
+
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
@@ -116,9 +118,9 @@ SidebarHeader.defaultProps = {
     teamType: ''
 };
 SidebarHeader.propTypes = {
-    teamDisplayName: React.PropTypes.string,
-    teamDescription: React.PropTypes.string,
-    teamName: React.PropTypes.string,
-    teamType: React.PropTypes.string,
-    currentUser: React.PropTypes.object
+    teamDisplayName: PropTypes.string,
+    teamDescription: PropTypes.string,
+    teamName: PropTypes.string,
+    teamType: PropTypes.string,
+    currentUser: PropTypes.object
 };