Remove globals from SpecialChat.class.php & linkify chat-no-login

The attached commits replace $wgOut and $wgUser in SpecialChat.class.php with the respective SpecialPage methods that return the same object, and fix the TODO present there, i.e. add a link to Special:Userlogin in MediaWiki:Chat-no-join and MediaWiki:Chat-no-join-text.