GH-4685 Post a System Message when channel is renamed (#4762)

* test changes

* added system message when displayname changed

* added test

* gofmt

webapp/components/rename_channel_modal.jsx

@@ -122,7 +122,7 @@ export class RenameChannelModal extends React.Component {
 
         const channel = Object.assign({}, this.props.channel);
         const oldName = channel.name;
-        const oldDisplayName = channel.displayName;
+        const oldDisplayName = channel.display_name;
         const state = {serverError: ''};
         const {formatMessage} = this.props.intl;