MM-11434 Only call PreparePostForClient once when creating a post (#9868) (#9876)

* MM-11434 Only call PreparePostForClient once when creating a post

* Have PreparePostForClient provide new metadata when a post already has it and update tests

app/notification.go

@@ -318,7 +318,10 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 	}
 
 	message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_POSTED, "", post.ChannelId, "", nil)
-	message.Add("post", a.PreparePostForClient(post).ToJson())
+
+	// Note that PreparePostForClient should've already been called by this point
+	message.Add("post", post.ToJson())
+
 	message.Add("channel_type", channel.Type)
 	message.Add("channel_display_name", notification.GetChannelName(model.SHOW_USERNAME, ""))
 	message.Add("channel_name", channel.Name)