Fixed MediaWiki logo in login sprite sheets.

Summary:
I've been advised that that the MediaWiki flower needs to
appear with the square brackets for proper MediaWiki trademark
presentation.

See https://phabricator.wikimedia.org/T543 for relevant discussion.

Test Plan:
view login page with MediaWiki oauth plugin enabled,
 see that the logo includes square brackets around the flower

Reviewers: epriestley, qgil, #blessed_reviewers, chad

Reviewed By: #blessed_reviewers, chad

Subscribers: Korvin, epriestley

Projects: #auth, #wikimedia

Differential Revision: https://secure.phabricator.com/D10752