Add --allow-admin-registration, useful for tests

BTCPayServer/Models/AccountViewModels/RegisterViewModel.cs

@@ -23,5 +23,8 @@ namespace BTCPayServer.Models.AccountViewModels
         [Display(Name = "Confirm password")]
         [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
         public string ConfirmPassword { get; set; }
+
+        [Display(Name = "Is administrator?")]
+        public bool IsAdmin { get; set; }
     }
 }