[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/actions/global_actions.jsx

@@ -225,6 +225,14 @@ export function showChannelHeaderUpdateModal(channel) {
     });
 }
 
+export function showChannelPurposeUpdateModal(channel) {
+    AppDispatcher.handleViewAction({
+        type: ActionTypes.TOGGLE_CHANNEL_PURPOSE_UPDATE_MODAL,
+        value: true,
+        channel
+    });
+}
+
 export function showGetPostLinkModal(post) {
     AppDispatcher.handleViewAction({
         type: ActionTypes.TOGGLE_GET_POST_LINK_MODAL,