Enable the Vector extension by default

This patch adds the Vector extension to 'role::mediawiki', ensuring it gets
installed by default. Vector is properly a part of MediaWiki, but some of its
code still lives in an extension for historic reasons. Bug 45051 tracks the
work of moving these parts into core.

Change-Id: Ic03bcf23d3166ab7492985b9b6850a604e40e090