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

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21661