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.