The Design Permission probably doesn't make sense any more

(Bug, Investigate , Priority: Critical, Test Status: No automated tests yet , Reported By Bad Link: Thing 3y284oe not found, )
Security
Summary: This needs sanity-checking.
Eric asked:
(Side question: For a Model, what's the difference between Edit rights and Design rights?)
The answer is that Design allows you to edit the Model, and Edit -- doesn't actually do anything.
This appears to be a historical artifact. It made sense once upon a time: Design applied to the Model itself, and Edit applied to the Instances. But then came the reorg of the Permissions system, which separated Model and Instance Permissions. At this point, I don't think there's any benefit from this distinction, and it adds confusion.
The hard part is going to be fixing up all of the existing Spaces: this looks like it requires an Evolution, which is always a bit tricky and fraught, so I'm not jumping right into it.