Ever since “The Notebook” made moviegoers swoon in 2004, Nicholas Sparks’ name has been synonymous with teary-eyed romance. The author’s latest novel adapted for the big screen, “The Longest Ride,” is no exception, offering two love stories at once. But the most heartfelt affair here isn’t between its impossibly good-looking stars; it’s shared by a couple 70 years their senior through a story told in flashbacks.