Hide marked empty elements by default (stage 2)

Hide marked empty elements by default (stage 2)

Follows-up d3d682fb45.

* Remove the obsolete mediawiki.raggett.css file and RL module. This requires
  the Varnish caches which reference it to have expired.
* Remove the mw-empty-li class from the CSS. This requires the parser
  cache entries which referenced it to have expired.
* Make the effect of mw-empty-elt be modifiable by user scripts, by
  switching it on only if the body class mw-hide-empty-elt is present.
  This requires the Varnish cache containing body elements without the
  relevant class to have expired.

Change-Id: Icdcff518be94b65c0dbbf6143a7ae4fb817b2182