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/common/comment_icon.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.
 
@@ -41,11 +43,11 @@ export default function CommentIcon(props) {
 }
 
 CommentIcon.propTypes = {
-    idPrefix: React.PropTypes.string.isRequired,
-    idCount: React.PropTypes.number,
-    handleCommentClick: React.PropTypes.func.isRequired,
-    searchStyle: React.PropTypes.string,
-    commentCount: React.PropTypes.number
+    idPrefix: PropTypes.string.isRequired,
+    idCount: PropTypes.number,
+    handleCommentClick: PropTypes.func.isRequired,
+    searchStyle: PropTypes.string,
+    commentCount: PropTypes.number
 };
 
 CommentIcon.defaultProps = {

webapp/components/common/post_flag_icon.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.
 
@@ -72,11 +74,11 @@ export default function PostFlagIcon(props) {
 }
 
 PostFlagIcon.propTypes = {
-    idPrefix: React.PropTypes.string.isRequired,
-    idCount: React.PropTypes.number,
-    postId: React.PropTypes.string.isRequired,
-    isFlagged: React.PropTypes.bool.isRequired,
-    isEphemeral: React.PropTypes.bool
+    idPrefix: PropTypes.string.isRequired,
+    idCount: PropTypes.number,
+    postId: PropTypes.string.isRequired,
+    isFlagged: PropTypes.bool.isRequired,
+    isEphemeral: PropTypes.bool
 };
 
 PostFlagIcon.defaultProps = {