Editing Choice Order Property lists all Things, not just the ones under this Model

Summary: See the Status Model in the Comics page. When I add Choice Order, I am getting all Things as my options, instead of just the Instances of Status as it should be.

See also the Grade Model. I think that, somewhere in building the new Client, we lost some crucial support for understanding how to display this, since it's a somewhat weird Link Property.

Note that the UI for this won't matter as much once we write the desperately-needed Choice Wizard, but this property will still be needed behind that.

Hmm. Looking at the code, I think this has always been broken in this respect. The issue at hand is going to go away when we add the Choice Wizard. And there's no obvious clean way to implement this yet, because Properties aren't scoped by their containing Thing. So I'm renaming this bug, and downgrading it slightly.

Later: actually, I think I have already built the necessary machinery. Look at Instance Properties -- this is the one use so far for LinkCandidateProvider, which appears to exist for precisely this purpose. Does it do the trick? Yep, that works. Lovely -- that's actually an elegant solution to the immediate problem.