Change entry point of SpecialPage

Change entry point of SpecialPage

The old entry point is the execute method, which clearly is not meant to be an entry point,
as it's overloaded in most subclasses, making it impossible for the deriving classes to do anything
before or after (which for instance sucks big time in SpecialCachedPage).

Change-Id: I74c22a1dc3b1a9d062295d5fb709fb87577810ae