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/spinner_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.
 
@@ -8,9 +10,9 @@ import loadingGif from 'images/load.gif';
 export default class SpinnerButton extends React.Component {
     static get propTypes() {
         return {
-            children: React.PropTypes.node,
-            spinning: React.PropTypes.bool.isRequired,
-            onClick: React.PropTypes.func
+            children: PropTypes.node,
+            spinning: PropTypes.bool.isRequired,
+            onClick: PropTypes.func
         };
     }