Gradebook hardcodes "default" skin in several RSF templates

Several RSF templates in gradebook hardcode references to /library/skin/default/tool.css instead of respecting the possible changes made by app administrators and site owners, particularly now that in 2.9 "default" is no longer the default skin. The preferred way to generate the CSS headers is by getting the HTML code from portal, as described in https://source.sakaiproject.org/svn//reference/trunk/docs/architecture/sakai_tool.doc.