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

* add /header slash command

* update websocket

* updater per review

webapp/utils/constants.jsx

@@ -176,6 +176,7 @@ export const ActionTypes = keyMirror({
     TOGGLE_GET_PUBLIC_LINK_MODAL: null,
     TOGGLE_DM_MODAL: null,
     TOGGLE_QUICK_SWITCH_MODAL: null,
+    TOGGLE_CHANNEL_HEADER_UPDATE_MODAL: null,
 
     SUGGESTION_PRETEXT_CHANGED: null,
     SUGGESTION_RECEIVED_SUGGESTIONS: null,