[PLT-6710] /rename [text] slash command: Rename the channel (#6570)

* /rename [text] slash command: Rename the channel

* update strings per review

webapp/utils/constants.jsx

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