Repository transfer has to be confirmed, if user can not create repo for new owner (#14792)

* make repo as "pending transfer" if on transfer start doer has no right to create repo in new destination

* if new pending transfer ocured, create UI & Mail notifications

modules/notification/mail/mail.go

@@ -170,3 +170,9 @@ func (m *mailNotifier) NotifyNewRelease(rel *models.Release) {
 
 	mailer.MailNewRelease(rel)
 }
+
+func (m *mailNotifier) NotifyRepoPendingTransfer(doer, newOwner *models.User, repo *models.Repository) {
+	if err := mailer.SendRepoTransferNotifyMail(doer, newOwner, repo); err != nil {
+		log.Error("NotifyRepoPendingTransfer: %v", err)
+	}
+}