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, but I think it's more important to have the reference to the associated organization; otherwise it's just a flag - @dino-cikatic, any opinions?