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 missed your latter comment, @martynjames -- so you'd rather have a nullable ForeignKey reference to Organization on the Project model, and let the client decide what to do when there's a value in that field?  So in the current use case it would interpret that value as being a 'private' project?