Implement one-to-many external edge

[schrockn]

Currently the only edge type supported is a stored id on object (which is canonical) and then (non-canonical) edges pointing from the other direction to create the many-to-one relationship. There should also be a variant which allows the id to be stored in a edge outside of the object. This will become clear if a mature system develops on this where there would be hundreds of relationships between a central object -- such as user -- and all the other features in the app.