DAMASCUS, SYRIA — Syria’s new rulers have appointed Murhaf Abu Qasra, a leading figure in the insurgency that toppled Bashar al-Assad, as defense minister in the interim government, an official source said on Saturday. Abu Qasra, who is also known by the nom de guerre Abu Hassan 600, is a senior figure in the Islamist Hayat Tahrir al-Sham group that led the campaign that ousted Assad …