[MM-16667] Migrate User.GetByUsername to Sync by default #11453 (#11520)

* Migrate User.GetByUsername to Sync by default #11453

* fix app errors #11453

* fix app errors #11453

* fix cmd error #11453

* fix shadow var #11453

* fix shadow user var #11453

* fix test error

* fix test error #11453

app/command_groupmsg.go

@@ -47,10 +47,9 @@ func (me *groupmsgProvider) DoCommand(a *App, args *model.CommandArgs, message s
 	for _, username := range users {
 		username = strings.TrimSpace(username)
 		username = strings.TrimPrefix(username, "@")
-		if result := <-a.Srv.Store.User().GetByUsername(username); result.Err != nil {
+		if targetUser, err := a.Srv.Store.User().GetByUsername(username); err != nil {
 			invalidUsernames = append(invalidUsernames, username)
 		} else {
-			targetUser := result.Data.(*model.User)
 			_, exists := targetUsers[targetUser.Id]
 			if !exists && targetUser.Id != args.UserId {
 				targetUsers[targetUser.Id] = targetUser