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_view.jsx

@@ -2,6 +2,7 @@
 // See License.txt for license information.
 
 import $ from 'jquery';
+import PropTypes from 'prop-types';
 import React from 'react';
 
 import * as UserAgent from 'utils/user_agent.jsx';
@@ -91,5 +92,5 @@ ChannelView.defaultProps = {
 };
 
 ChannelView.propTypes = {
-    params: React.PropTypes.object.isRequired
+    params: PropTypes.object.isRequired
 };