Yet more factory-related stuff

This commit is a mish-mash of improvements, none of which are terribly exciting, but all of which are useful.

1. Improve API documentation interlinks. In several places, replace class and function names with actual hyperlinks to that class or function.
2. Add two new entities: `Organization` and `Product`. Add two corresponding factories.
3. Make the `Factory` object correctly handle foreign keys. When a factory is asked to generate attributes for an entity, and that entity includes a required foreign key to some other entity, the factory will now provide a value of `None` for that field. This behaviour will be very useful to the `create` method, or to any API unit test that wants to manually assemble object attributes before talking to an API URL.

Following on the heels of this PR, I'm going to immediately start writing more tests. Most of the factory framework has now been built up, and @AdamSaleh has expressed interest in implementing method `Factory.create`, so making use of this infrastructure is probably a good thing for me to be doing.