[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/stores/modal_store.jsx

@@ -42,6 +42,7 @@ class ModalStoreClass extends EventEmitter {
         case ActionTypes.TOGGLE_DM_MODAL:
         case ActionTypes.TOGGLE_QUICK_SWITCH_MODAL:
         case ActionTypes.TOGGLE_CHANNEL_HEADER_UPDATE_MODAL:
+        case ActionTypes.TOGGLE_CHANNEL_PURPOSE_UPDATE_MODAL:
             this.emit(type, value, args);
             break;
         }