Set enwiki's languageLinkSiteGroup to 'wikipedia'

Since I4e71671c8, WikibaseClient's OutputPageParserOutput & ParserAfterParse
hook handlers call WikibaseClient::getLangLinkSiteGroup(). Since
$wgWBClientSettings['languageLinkSiteGroup'] is unset, it defaults
to WikibaseClient::getSite, which calls SiteSQLStore::getSites, which requires
retrieving a huge object from memcached, with a predictable impact on the
cluster: see <http://noc.wikimedia.org/~ori/SiteSQLStore.html> -- you can guess
when I4e71671c8 was deployed.

This patch sets $wgWBClientSettings['languageLinkSiteGroup'] for enwiki to
validate my investigation of this issue and to provide a temporary workaround
for the bug 56602.

Bug: 56602
Change-Id: Id33ec93666b03469082f14ec2b87eb064fed1928