CAIRO — Egypt sent a submarine Sunday to join the hunt for the flight recorders from the EgyptAir jetliner that crashed in the Mediterranean and killed all 66 people aboard, while hundreds of Coptic Christian mourners filled a church in Cairo to pray for their relatives among the dead. Beside Egypt, ships and planes from Britain, Cyprus, France, Greece and the United States are taking part in the search for the debris from the aircraft, including its flight data and cockpit voice recorders. Egypt’s aviation industry has been under international scrutiny since Oct. 31, when a Russian Airbus A321 traveling to St. Petersburg from the Red Sea resort of Sharm el-Sheikh crashed in the Sinai Peninsula, killing all 224 people aboard. The nation of 90 million people has been in crisis after crisis since a popular 2011 uprising toppled autocrat Hosni Mubarak. [...] it has seen a dramatic surge in attacks by Islamic militants, bouts of deadly unrest, a battered economy and the steady decline in the value of its currency.