"Create a New Thingy" in Link selection has been long broken

Summary: When you have a Required/Optional Thing Property, the drop-down includes an option "Create a New whatever". This has been broken for, literally, years.

This feature got broken when we went to the real Client, because I never reimplemented it.

In practice, I've come to believe that this feature isn't especially desireable to begin with: if this is what you want, you should be using Tags instead. (Indeed, from a workflow POV this is exactly the difference between a Tag Property and a Thing Property.)

We might revive this eventually in some form, but for now I'm going to just comment it out.