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.
I'm not sure we want to default this field to True -- @martynjames @dino-cikatic, can you comment on which value makes more sense for the default?