Calling all conspiracy theorists! The National Archives is set to release the last remaining top-secret files about John F. Kennedy’s 1963 assassination, in a process that could begin by this summer. The trove of some 3,600 files, mostly from the FBI and CIA, were part of the collection assembled and sealed by the Archives, on the condition that they all be made public by October 2017. But there’s a catch: According to the same law, President Donald Trump has the ability to block the release of any or all of the documents—if he certifies that keeping them secret is a...