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.
So, the link background property url should look like this, if I put it into the ExtraHeader style tag:
``` css
background: url(<? VAR URIPrefix TOP ?>/skinfiles/_default_/External.png) center right no-repeat;