Various Userrights-related fixes:

Various Userrights-related fixes:
* Adjust UserrightsForm so that it inherits from SpecialPage; nuke HTMLForm.  Since this breaks backward compatibility, renamed to UserrightsPage.
* Created SpecialPage::isRestricted() and enforced use of SpecialPage::userCanExecute() instead of hardcoded checks.  These can now be overridden so that more complicated restriction systems work sanely.  Used them for UserrightsPage (fixes bug 12489).
* A few random comment/documentation tweaks.
Also, update Special:Version date.