I can't allow only some Members access to the Space Thing

Summary: This is a side-effect of the fact that we only check Roles if the permission is not set on this specific Thing. So if I restrict Space to Owner-only, Collaborators still can't see it, because the Space Thing says not to.

This is essentially the same problem that led to the creation of Who Can Edit Children, and I think it's the straw that forces us to generalize that. The only real question is to solve this just for permissions, or to do so for all Properties. Default View (for Instance vs. Model) has shown that we might want to do it more generally, but that's a big across-the-board expense, so I'd prefer to keep this more specific unless I'm sure that there are more examples.

The solution is likely that a Permission can point to another Permission, with a new Property for Instances link. When we look for Permission A on Thing T, we first check whether that specific permission is set on T; if not, we look at linked Permission A', if it exists for this Permission. If so, we check whether that Permission is set on T or any of the ancestors of T; otherwise, we check whether A is set on the ancestors.

Does this solve the problem? Think it through carefully. Also, consider renaming the existing Properties, to emphasize that the Instances version is primary, and should usually be used.