changed old link icons to use icons from same icon set as sitetools

lib/tpl/dokuwiki/css/_links.css

lib/tpl/dokuwiki/images/email.png

lib/tpl/dokuwiki/images/external-link.png

lib/tpl/dokuwiki/images/icons-license.txt

lib/tpl/dokuwiki/images/link_icon.gif

lib/tpl/dokuwiki/images/mail_icon.gif

lib/tpl/dokuwiki/images/unc.png

lib/tpl/dokuwiki/images/windows.gif