Break logo/name into replaceable parts

Summary: Ref T4214, this breaks the 'eye' out as a separate image 40px x 40px. We also now show the eye on mobile, as we have enough room for both currently.

Test Plan: Tested default and nightmaremoon colors, tested mobile, tablet and desktop layouts.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4214

Differential Revision: https://secure.phabricator.com/D10794