A senior cleric with Nigerian Islamist group Boko Haram confirmed the death of its leader and urged fighters to stay loyal in its fight against rival Islamic State militants, according to a video seen by Reuters. Bakura Sahalaba, a cleric with the Lake Chad branch of Boko Haram, urged its men to fight on in the video, which appeared on social media after the rival Islamic State West Africa Province (ISWAP) said Boko Haram leader Abubakar Shekau had died. Three Boko Haram researchers verified its authenticity. ISWAP had said Shekau, one of the world's most notorious extremists, had died around…