[PLT-6708] /purpose [text] slash command: Edit the channel purpose (#6569)

* /purpose [text] slash command: Edit the channel purpose

* update command on server side to check for direct or group channels

* update stings and block the dialog when is DM or GM

* update per review

* remove duplicate websocker event and apply the same for /header command

* update per review

* update

webapp/components/create_post.jsx

@@ -227,12 +227,19 @@ export default class CreatePost extends React.Component {
             return;
         }
 
-        if (this.state.message.endsWith('/header ')) {
+        if (this.state.message.trimRight() === '/header') {
             GlobalActions.showChannelHeaderUpdateModal(updateChannel);
             this.setState({message: ''});
             return;
         }
 
+        const isDirectOrGroup = ((updateChannel.type === Constants.DM_CHANNEL) || (updateChannel.type === Constants.GM_CHANNEL));
+        if (!isDirectOrGroup && this.state.message.trimRight() === '/purpose') {
+            GlobalActions.showChannelPurposeUpdateModal(updateChannel);
+            this.setState({message: ''});
+            return;
+        }
+
         this.doSubmit(e);
     }