/// <summary>
 /// 標準的なGravatar用のクラスを取得する
 /// </summary>
 /// <param name="user"></param>
 /// <returns></returns>
 public static BasicGravatarLoader GetBasicGravatarLoader(UserAccount user)
 {
     return new BasicGravatarLoader(user.Email);
 }
 private async Task SignInAsync(UserAccount user, bool isPersistent)
 {
     AuthenticationManager.SignOut(DefaultAuthenticationTypes.ExternalCookie);
     AuthenticationManager.SignIn(new AuthenticationProperties { IsPersistent = isPersistent },
         await user.GenerateUserIdentityAsync(UserManager));
 }