Add 'mediaWikiLoadComplete' measurement

Log the time from mediaWikiLoadStart to first script execution yield following
$( window ).load. Depends on changes I3d5bcf10e and I8c7af097e in MediaWiki.
Also bumps schema version to 6703470.

Change-Id: I933a1e3a20c1c2e03955c310ff62d9b8b44ca7d1