public async Task <ActionResult> RefreshToken(string returnUrl)
        {
            await SharePointAuth.EnsureClientCreated();

            Response.Redirect(Server.UrlDecode(returnUrl));
            return(View());
        }
        public async Task <ActionResult> Login()
        {
            try
            {
                var client = await SharePointAuth.EnsureClientCreated();

                Response.Redirect("/index.html");
                return(View());
            }
            catch (RedirectRequiredException ex)
            {
                return(Redirect(ex.RedirectUri.ToString()));
            }
        }