OutputPage: Move hardcoded default modules to Skin::getDefaultModules
These modules should not be hardcoded in OutputPage::output() which
makes them impossible to override and also very hard to retrieve
through the API for action=parse. Move these instead to Skin which
is where all other default module loading happens already.
Moving these modules is in preparation for customising ApiParse
to support "really" returning all would-be loaded modules on a page
when setting 'useskin', which is also needed for Live Preview
and in theory for ajax navigation and other scenarios where there
is a delay between the "initial" page rendering, and a later re-render
which may not have all all the necessary modules.