SpecialStatistics: Implement statistics-articles-desc message

SpecialStatistics: Implement statistics-articles-desc message

Similar to the statistics-pages-desc message that exists already.
This way mediawiki.org can stop using the hack currently employed
in the local override for the 'statistics-articles' message which
adds a line break inside the link label to emulate a caption. This
doesn't work well because it breaks after the first link.

Also:
* Change from exists() to !isDisabled() so that it can be optional.
* Fix wrong use of Xml::element and use Html::rawElement instead.
  The message in question is already escaped. This was causing
  escaped parser html to render as text on the page when the desc
  message contains e.g. a wiki link.

Change-Id: I547d1da16c8bdbabddad35525a5c9fa34b8318d4