Introducing special page modular extensions, making the board vote special page the...

Introducing special page modular extensions, making the board vote special page the first of these.

* Moving common special page handling to SpecialPage.php
* Special page lists in the language classes are obsolete, replaced by the list in SpecialPage.php
* Special:Specialpages uses the standard page titles wfMsg(pagename) instead of the description list previously contained in the $wgSpecialPagesEn arrays
* Ability to add custom messages to the MessageCache. Overridable by the MediaWiki namespace.
* Moving board vote functionality from includes/SpecialBoardvote.php to extensions/BoardVote.php