Skin: Reduce database queries for footer links on every page

Skin: Reduce database queries for footer links on every page

When viewing Special:Blankpage there are still 6 database queries
on every page view. 3 of these are from the Skin:

	> $tpl->set( 'disclaimer', $this->disclaimerLink() );
	> $tpl->set( 'privacy', $this->privacyLink() );
	> $tpl->set( 'about', $this->aboutLink() );

In Wikimedia production, Xenon flame graphs (reversed) for index.php
attribute 22% of LinkCache::fetchPageRow calls to Skin::footerLink().

Add them to Skin::preloadExistence() instead.

Change-Id: I61c285be08a2130fb39b75ca717ea83f297c4489