Cities around the globe are facing a stark reality: There are not enough affordable places for people to live. Housing in most big urban areas is now considered out of reach for the average worker, fueling a growing sense that a decent home has become a privilege traded among the “haves,” impoverishing the “have-nots.” Thorny questions abound: Should new luxury apartments in London — many owned by foreigners — be allowed to lie empty, while thousands of people sit on waiting lists for affordabl