BEIRUT — In a brazen assault near the Syrian capital, Islamic State militants abducted 300 cement workers and contractors from their workplace northeast of Damascus on Thursday, as fighting against the extremist group raged in the country’s north ahead of a new round of peace talks. In a blow to the Islamic militants, however, rebel fighters wrested control of a northern border town, potentially undermining IS supply lines across the border with Turkey and endangering one of its most important strongholds in Aleppo province. The U.N.