PARIS — If you post a 19th-century nude painting on Facebook, is it art or impermissible nudity? That question is now cleared for trial in France, after an appeals court there ruled that an aggrieved user can sue the social network over the issue.Five years ago, Facebook suspended the account of Frederic Durand-Baissas, a 57-year-old Parisian teacher and art lover, without prior notice. That was the day he posted a photo of Gustave Courbet’s 1866 painting “The Origin of the World,” which depicts female genitalia.Durand …