moved plugin retrieval to Lesson.get_sourceDB_plugin() classmethod
moved plugin retrieval to Lesson.get_sourceDB_plugin() classmethod

cjlee112/socraticqs2,raccoongang/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,raccoongang/socraticqs2,derdmitry/socraticqs2,cjlee112/socraticqs2,derdmitry/socraticqs2,cjlee112/socraticqs2,raccoongang/socraticqs2,derdmitry/socraticqs2