DAKAR, Senegal — When nearly 300 Nigerian schoolgirls were kidnapped from their boarding school by the Islamist group Boko Haram in 2014, the world exploded in outrage. Hundreds marched in the country’s capital, the hashtag #BringBackOurGirls was picked up by then First Lady Michelle Obama and Nigeria’s president scrambled to respond to the mass abduction in the village of Chibok.