Move installer files from skins/common/ to mw-config/

Move installer files from skins/common/ to mw-config/

The CSS and JS files are definitely used only by the installer.

As for the images:
* mediawiki.png is still used directly by some error pages
  (includes/PHPVersionError.php, includes/templates/NoLocalSettings.php)
* ajax-loader.gif is still used by shared.css
* bullet.gif is mysteriously used by *something*, according to the
  logs at bug 69277 comment 11; I currently have no idea what, so
  let's keep it here for a while
* All other ones don't grep (outside of the installer itself) and
  don't appear in the logs on bug 69277.

Bug: 69277
Change-Id: I9146d9211a807911a5e0cfaa1dd3ab8170f333ca