Install Vector skin when installing MediaWiki

Idfc38503 removes Vector from mediawiki/core which makes the default
MediaWiki installation unusable until a skin is installed separately.

This patch adds a mediawiki::skin class to check out a skin and
require it; and uses that class to install Vector by default.

Change-Id: Ic321a26b5c37b39bb97f99f0ba8a35dd491c6376