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