made Concept.get_url() prefer UnitLesson in this unit
made Concept.get_url() prefer UnitLesson in this unit

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