Replace "Phabricator" menu text with phabricator logo

Summary:
I ran a 99designs contest and this gear-eye thing was actually pretty okay. All
this stuff needs tweaks but at least it won't render with a big square on
windows anymore.

Test Plan:
Looked at menu, it seemed slightly more legitimate and designey than before?

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, epriestley, jungejason
Differential Revision: 363