Move some member variable initialization to the declarations and out of constructors...

Move some member variable initialization to the declarations and out of constructors.  This is better style and less error-prone: OutputPage::$mIsPrintable was first being set to true and then to false in the constructor, and $mAllowUserJs and $mLinkColours are missing defaults entirely.  Also, Tim said there's some lazy-loading or shared memory or something if it's done this way, so it might be a performance boost.