The 50-year-old company, a legend of the late 20th-century downtown avant-garde theater scene, is back at the old P.S. 122 building, just renovated, in Manhattan’s East Village. Reporter Zachary Small visits the new Mabou Mines HQ and talks with co-artistic director Sharon Fogarty and company co-founder Lee Breuer, who – at age 80 – is premiering a new play there.