Refactor Story and Comment validators

[mattrobineau]

Instead of validating users in the story and comment validators eg:

var user = Task.Run(async () => await UserService.GetUser(instance.UserId)).Result;

            if (user == null)
            {
                result.IsValid = false;
                result.Messages.Add("Error: Please sign in again.");
            }

            if (user.IsBanned)
            {
                result.IsValid = false;
                result.Messages.Add("You account is banned.");
                return result;
            }

Validate the users using a user specific validator in the controller actions.