The bleakly beautiful and intensely nerve-rattling “1917” (opening Friday) ostensibly is a war film set amid the brutal realities of trench warfare between England and Germany on the fields of rural France in World War I. And it is that, so much so that you can almost feel the mud and viscera ooze between your fingers when a soldier, who has landed in a pool of muck, mistakes a fallen fighter’s corpse for something sturdy and his hand plunges deep into a body cavity. But “1917,” the latest film from director Sam Mendes (who made the James Bond films “Skyfall”...