FEATURE: Allow admins to access group chats (#31440)

In #31366, we added the ability for admins to remove people from group chats. However, that only works as long as the admin is already in the group chat.

For forum-side group messages, admins can join any of them at will. This PR extends that same ability to chat for parity.

plugins/chat/app/models/chat/direct_message.rb

@@ -27,7 +27,7 @@ module Chat
     end
 
     def user_can_access?(user)
-      users.include?(user)
+      users.include?(user) || group? && user.admin?
     end
 
     def chat_channel_title_for_user(chat_channel, acting_user)