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_dropdown_button.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.
 
@@ -7,8 +9,8 @@ import Constants from 'utils/constants.jsx';
 
 export default class SidebarHeaderDropdownButton extends React.PureComponent {
     static propTypes = {
-        bsRole: React.PropTypes.oneOf(['toggle']).isRequired, // eslint-disable-line react/no-unused-prop-types
-        onClick: React.PropTypes.func.isRequired
+        bsRole: PropTypes.oneOf(['toggle']).isRequired, // eslint-disable-line react/no-unused-prop-types
+        onClick: PropTypes.func.isRequired
     };
 
     render() {