FIX: Make ChatMessageUpdater check editing access for guardian (#18902)

Follow up to 766bcbc6840c9d665055441bcd77616b3a96e10e

This fixes a gaffe from that commit where I passed in the
guardian to ChatMessageUpdater but then forgot to remove
the old way of setting the guardian and user instance variables
from the chat_message that was passed in.

Also, it moves the ensure_can_edit_message! check from the
controller into ChatMessageUpdater so all the access
checks are in the same place.

plugins/chat/app/controllers/chat_controller.rb

@@ -144,7 +144,6 @@ class Chat::ChatController < Chat::ChatBaseController
   end
 
   def edit_message
-    guardian.ensure_can_edit_chat!(@message)
     chat_message_updater =
       Chat::ChatMessageUpdater.update(
         guardian: guardian,