mediawiki.skinning.content.externallinks: Code quality tweaks

mediawiki.skinning.content.externallinks: Code quality tweaks

* Use the standard SVG+PNG background-image method, used throughout
  MediaWiki. The one used previously was very similar, but did not
  support Android 2.3 (neither PNG nor SVG displayed) which is
  allegedly still big on mobile devices. The current one supports
  every browser in existence, but sometimes serves PNG to browsers
  that understand SVG (e.g. IE 9).
* Add a note about why we can't just use LESS mixins for that.
* Remove incorrect code comments and tweak whitespace.

Change-Id: I2bedfd3901b35c1cc1a7b37db8f4af6bb5e4cc80