After days of protests in Chicago since the release of a video showing a black teenager shot 16 times by a white police officer, demonstrators were poised to disrupt Black Friday shopping with a march through the heart of the city’s most famous retail district.Demonstrators scheduled the march for today, the traditional beginning of the holiday shopping season that packs Michigan Avenue’s Magnificent Mile. Activists hope to bring attention to the Oct. 20, 2014, slaying of 17-year-old Laquan McDonald and an …