Marley Morrison’s knockout debut feature is a sharply observed coming-of-age story that will make you wince and laugh in painful recognition of universal truths. 17-year-old AJ (Nell Barlow) is not one of life’s shiny, happy people. A family holiday at a caravan park in Dorset is her idea of hell. A moody misfit, she dresses for concealment and lets everyone know that she would…