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/system_users/system_users.jsx

@@ -1,3 +1,5 @@
+import PropTypes from 'prop-types';
+
 // Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
@@ -34,10 +36,10 @@ const USERS_PER_PAGE = 50;
 
 export default class SystemUsers extends React.Component {
     static propTypes = {
-        actions: React.PropTypes.shape({
-            getTeams: React.PropTypes.func.isRequired,
-            getTeamStats: React.PropTypes.func.isRequired,
-            getUser: React.PropTypes.func.isRequired
+        actions: PropTypes.shape({
+            getTeams: PropTypes.func.isRequired,
+            getTeamStats: PropTypes.func.isRequired,
+            getUser: PropTypes.func.isRequired
         }).isRequired
     }