mattdrayer/api-workgroups: Added new Workgroup entity

@ormsbee, here's a very basic set of models and views for the Workgroup entity concept we discussed -- no relationship tables yet -- we can discuss those more as we go -- would like to get your feedback on this PR, move to a more standard DRF model/entity serializer, as well as possibly utilize ViewSets here since it's a "pure" Django model we're dealing with in this case.

@martynjames, @chrisndodge, FYI