Fixes issue #505: Module links in webadmin should use _blank.

[Wikipedia Icon External Link](http://commons.wikimedia.org/wiki/File:Icon_External_Link.png) is being used as an icon for the external link.
If you're talking about `<? ADDROW CSSLoop HREF=/skinfiles/_default_/global.css ?>`, it becomes `<link rel="stylesheet" type="text/css" href="/foo/skinfiles/_default_/global.css" />` in BaseHeader.tmpl if URIPrefix is `/foo`.
But these static CSS files don't go through template engine and have no chance to replace `<? VAR URIPrefix TOP ?>` with anything...
Currently there are two types of files: templates, and static files. Probably we should have a better way to put such stuff to CSS (make it less static)
Perhaps the easiest way now would be to put `<style>` to ExtraHeader.tmpl which is included from BaseHeader.tmpl too