CHICAGO — Nowin theaters, “Till” is the story of a mother, a mother’s son and the horrific 1955 lynching of 14-year-old Chicagoan Emmett Till, who was visiting relatives in Mississippi at the time of his murder. The killers were acquitted and lived as free men, long after confessing, proudly, to Look magazine the following year. The decision of Mamie Till-Mobley, Emmett’s grieving, quietly galvanized mother, to hold an open-casket funeral for her son, and to allow Chicago’s Jet magazine to publish photographs of Emmett Till’s brutalized body, sparked a civil rights movement. Generations later,…