Flatten Phabricator logo to white

Summary: Update logo/icon to flat white. I beleive this is the last of the 'chrome' iconography.

Test Plan:
Internally debate in my head the correct path. Place up for review with the Phabricator Reviewer Gods.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11561