The opening subtitle, “Based on a true story,” can cover a multitude of movie sins, but in “A United Kingdom,” it unlocks the door to a romantic drama that grows more remarkable by the minute. While lovers faced with daunting obstacles is a dramatic tradition going back to Romeo and Juliet, if not further, the real-life barriers facing Seretse Khama and Ruth Williams when they fell in love in 1947 London were unusually intimidating.