MOGADISHU, Somalia — Islamic extremists controlling part of the Somali capital of Mogadishu in November executed a young Christian they accused of trying to convert a 15-year-old Muslim to Christianity. Members of the Islamic extremist group al Shabaab had taken 23-year-old Mumin Abdikarim Yusuf into custody Oct. 28 after the 15-year-old boy reported him to the militants, an area source…