Reduce a bit the coupling between Skin and OutputPage:

Reduce a bit the coupling between Skin and OutputPage:
* Removed Skin::setupUserCss() and merged it in OutputPage::buildCssLinks() and OutputPage::buildCssLinksArray(), also removed its call from ApiParse.php
* Moved Skin::userCanPreview() to OutputPage since two of the three calls to that function are made from there and the last one in SkinTemplate.php is in a somewhat deprecated function
* Removed the Skin parameter from OutputPage::buildCssLinks(), OutputPage::getHeadScripts(), OutputPage::getBottomScripts() and OutputPage::makeResourceLoaderLink() since we now have a context

Also made ApiParse.php call createContext() instead of create a new RequestContext manually and set its Title  so so that it does not rely on $wgRequest and $wgTitle to get that member objects