Ziafazal/users in workgroup

Add is_private field to workgroup model to support MCKIN-1414 and added new api to list workgroups of user with course_id filter to support MCKIN-1425.
@mattdrayer Please review.
Yes, I'd say False for the default; but really it's the project that is private so I'm not sure that this field even belongs here. Of course, without editing studio there is now easy way to set this, so perhaps this is how it has to work for now?