Some misc fixes to Skin and SkinTemplate:

Some misc fixes to Skin and SkinTemplate:
* Removed Skin::getStylesheet(), not called from anywhere
* Removed Skin::setMembers() and moved the definition of the $userpage member to SkinTemplate::outputPage() since it's only used in that class
* Removed useless $out parameter of Skin::bottomScripts()
* Call Linker methods statically
* Use Linker::linkKnown() instead of Linker::link() if possible
* Escape 'mainpage' message when used for description
* Removed some variables from SkinTemplate::outputPage(): nscanonical, nsnumber, currevisionid that were used when the JavaScript global variables were defined in the template. Also changed pageclass and skinnameclass to be defined only when $useHeadElement is false
* Removed some unused members of SkinTemplate: $iscontent and $iseditable
* Refactored the part of SkinTemplate::outputPage() that sets copyright, viewcount, lastmod, credits and numberofwatchingusers to always define them to false first and set them when needed. Also modified the check to display the copyright; it will now only be displayed when $out->isArticle() is true and the page exists, since it is currently possible to display it on any page by adding the oldid parameter to the url.