Rework new favicon

Summary: This makes all favicons a background circle w/color and white gear, to be less confusing to an actual gear icon.

Test Plan:
review many different versions in sandbox.

{F1811707,size=full}

Reviewers: epriestley, avivey

Reviewed By: avivey

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16539