public ViewResult RsvpForm(Response response)
 {
     response.Events = repositoryEvent.GetEvent(response.EventID);
     if (ModelState.IsValid)
     {
         if (!repository.IsRegistered(response))
         {
             repository.AddResponse(response);
             return(View("Registered", response));
         }
         else
         {
             TempData["message"] = $"{response.Email} is already registered to this event.";
             return(View(response));
         }
     }
     else
     {
         return(View(response));
     }
 }