Pull back r56730 for now: "New configuration variable $wgShowPageOnRedlink that can...

Pull back r56730 for now: "New configuration variable $wgShowPageOnRedlink that can be set to show the page instead of an edit interface when visiting a red link. The value can be specified for specific usergroups and namespaces."
The config var's formatting strikes me as kinda scary; might be better to implement this as a permission key, which can have fancy namespace variants handled by an extension or something pretty easily (it seems to me like that's a rarer use case).