* Reintroduce user page move permission as per r41465, generally useful regardless...

* Reintroduce user page move permission as per r41465, generally useful regardless of whether the Renameuser extension is in use. Do the check in Title::getUserPermissionsErrorsInternal(), so that the move page tab won't be displayed on user pages for users who can't move them.
* Use error message text suitable for humans. Be specific about whether the source or the destination is the problem, say what the namespace is for those people who don't get namespaces, don't introduce unnecessary jargon words like "root" for a page that isn't a subpage.
* Fixed incorrect display of movenotallowed for immobile namespaces, use the new specific error messages.