New MediaWiki logo for the icon that represents the MediaWiki oauth provider.
Summary: Replaced the old logo with a new png.
Test Plan: I only tested that the new image was the same dimensions, format and filename as the old one.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Differential Revision: https://secure.phabricator.com/D21661