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/channel_select.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.
 
@@ -11,11 +13,11 @@ import {sortChannelsByDisplayName} from 'utils/channel_utils.jsx';
 export default class ChannelSelect extends React.Component {
     static get propTypes() {
         return {
-            onChange: React.PropTypes.func,
-            value: React.PropTypes.string,
-            selectOpen: React.PropTypes.bool.isRequired,
-            selectPrivate: React.PropTypes.bool.isRequired,
-            selectDm: React.PropTypes.bool.isRequired
+            onChange: PropTypes.func,
+            value: PropTypes.string,
+            selectOpen: PropTypes.bool.isRequired,
+            selectPrivate: PropTypes.bool.isRequired,
+            selectDm: PropTypes.bool.isRequired
         };
     }