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.jsx

@@ -22,14 +22,16 @@ import {Dropdown} from 'react-bootstrap';
 import {FormattedMessage} from 'react-intl';
 import {Link} from 'react-router/es6';
 
+import PropTypes from 'prop-types';
+
 import React from 'react';
 
 export default class SidebarHeaderDropdown extends React.Component {
     static propTypes = {
-        teamType: React.PropTypes.string,
-        teamDisplayName: React.PropTypes.string,
-        teamName: React.PropTypes.string,
-        currentUser: React.PropTypes.object
+        teamType: PropTypes.string,
+        teamDisplayName: PropTypes.string,
+        teamName: PropTypes.string,
+        currentUser: PropTypes.object
     };
 
     static defaultProps = {