Ref T10010. Three motivations:
- Primarily: this makes conversion to EditEngine easier since I don't have to convert this weird control.
- This probably needs to have "Lock", "Unlock" and "Use Parent Project Setting" values after subprojects? But maybe just locking any parent locks all the children? Anyway, doesn't make sense to put it on the main edit form if it's weird like this, I think, since we'll want some kind of explanatory text.
- I probably want to move this to the "Members" tab anyway, and this won't be available on milestone projects at all.