NEW YORK — A subway tunnel that was severely damaged by Superstorm Sandy will be closed for 18 months starting January 2019 for repairs, shutting down one of the system’s most crowded lines that connects Manhattan with vibrant neighborhoods in Brooklyn, officials said Monday. The Metropolitan Transportation Authority decision means places like Williamsburg and Bushwick that had thrived in recent years as draws for young people, arts and nightlife will have their East River transportation lifeline to Manhattan severed. The MTA chose to close the Canarsie Tube completely for a year and a half instead of a three-year partial shutdown.