public IActionResult Add(AddUserViewModel addUserViewModel)
        {
            if (ModelState.IsValid)
            {
                User newUser = addUserViewModel.NewUser(
                    addUserViewModel.Username,
                    addUserViewModel.Email,
                    addUserViewModel.Password
                    );

                UserData.Add(newUser);

                return(Redirect("/"));
            }

            return(View(addUserViewModel));
        }