[PLT-6710] /rename [text] slash command: Rename the channel (#6570)

* /rename [text] slash command: Rename the channel

* update strings per review

webapp/components/create_post.jsx

@@ -243,6 +243,12 @@ export default class CreatePost extends React.Component {
             return;
         }
 
+        if (!isDirectOrGroup && this.state.message.trimRight() === '/rename') {
+            GlobalActions.showChannelNameUpdateModal(updateChannel);
+            this.setState({message: ''});
+            return;
+        }
+
         this.doSubmit(e);
     }