Mailsender hardcodes "default" skin in several RSF templates

In several RSF templates, references to /library/skin/default/tool.css. Instead, this should be generated to respect the settings of site owner and app administrator, especially since "default" is no longer the default skin for Sakai as of 2.9. The preferred way to retrieve the appropriate CSS headers is from the portal as described in https://source.sakaiproject.org/svn//reference/trunk/docs/architecture/sakai_tool.doc on line 300.