[PLT-6707] /header [text] slash command: Edit the channel header (#6566)

* add /header slash command

* update websocket

* updater per review

webapp/stores/modal_store.jsx

@@ -41,6 +41,7 @@ class ModalStoreClass extends EventEmitter {
         case ActionTypes.TOGGLE_GET_PUBLIC_LINK_MODAL:
         case ActionTypes.TOGGLE_DM_MODAL:
         case ActionTypes.TOGGLE_QUICK_SWITCH_MODAL:
+        case ActionTypes.TOGGLE_CHANNEL_HEADER_UPDATE_MODAL:
             this.emit(type, value, args);
             break;
         }