Bug 46470: Expose {{DEFAULTSORT}} as meta

Patch by Toby Hutton and tweaked by Subbu and GWicke.

* Implements the DOM spec at
  http://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#Category_default_sort_key

* Round-trips default sort keys using the original wiki source, but does not
  support editing yet.

No changes in parserTests.

TODO:
* Add editing support
* Possibly clean up slightly

Change-Id: I0d4c2b3d087fba700f91c2714cec5b8933882ca0