Enterprises (and other AbstractAgents) can be affiliates of an Enterprise

In the current version of this package an Enterprise's affiliates can currently only contain instances of the model Person, but they should be able to contain Enterprise instances also. For example, a Locavora producer should be affiliated to the Locavora hub where it sells its produce.

We can add dfc-b:affiliatedTo on Enterprise without conflicts, but currently DjangoLDP's serialization does not support containers which contain instances of different models, so we couldn't serialize everything under the Enterprise's dfc-b:affiliates without also improving DjangoLDP: djangoldp#373