When runnin w/in Rails, make available url_helpers

This technique ties this usage to Rails, which but makes the helpers
directly accessible w/in any instance of ActiveModel::Serializer,
including any associations.
Use Category for nesting hypermedia test data

The Hypermedia* models and serializers didn't were a bit nonsensical,
and thus hard to reason about. Category seems to fit the domain of the
other test models.