MM-11434 Performance improvements for post metadata (#9849)

* Remove unused error return value from PreparePostForClient

* Remove unused error return value from PreparePostListForClient

* MM-11434 Parallelize PreparePostListForClient

* MM-11434 Skip looking reactions and files on post whenever possible

* Add note about the use of deprecated fields

app/notification.go

@@ -317,13 +317,8 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		}
 	}
 
-	clientPost, err := a.PreparePostForClient(post)
-	if err != nil {
-		mlog.Error("Failed to prepare new post for client", mlog.Any("err", err))
-	}
-
 	message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_POSTED, "", post.ChannelId, "", nil)
-	message.Add("post", clientPost.ToJson())
+	message.Add("post", a.PreparePostForClient(post).ToJson())
 	message.Add("channel_type", channel.Type)
 	message.Add("channel_display_name", notification.GetChannelName(model.SHOW_USERNAME, ""))
 	message.Add("channel_name", channel.Name)