Tag Models shouldn't show in the Things section, by default

Summary: Currently, in the default Space Root page, we show all local models. This includes Tag Models, as well as Choice Models. It probably shouldn't show either.

We really should filter these out of the view by default -- there might be a small "Other Models" link down below to see them, but they shouldn't be clogging up the view.

The heart of the problem is that we don't yet have an "Is a Tag Model" property, corresponding to Is a Choice. Nor would we want the end user to be adding these by hand.

The implication is that, as part of the workflow of creating a Tag Set property, we should automatically create a matching Model. It might be optional, but it should happen by default, since it is usually what we want. The current workflow is a hassle, and we should move away from it. Once the process of creating this Model is automated, we can more easily screen it out of the standard Things view.