Update Phabricator logo

Summary:
This updates the eye logo and removes the formal wordmark "Phabricator" as an image. Instead we'll use the new updated eye logo and plain text for "Phabricator", both of which are more friendly and less industrial.

Installs that already use the header-logo customization setting will need to rebuild their logo to 80px x 80px. They will then also get to use plain text to whitebox their install as they see fit.

Test Plan:
Tested new logo at desktop, tablet, and mobile sizes. Set a random instance name, saw new wordmark. Created a really long wordmark of MMMMMMMMMMMM, saw text cut off so UI doesn't break. May need some additional tweaking, but I think we covered the most edge cases here.

{F1751791, size=full}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: edibiase, bjshively, yelirekim, Korvin

Maniphest Tasks: T4214, T11096

Differential Revision: https://secure.phabricator.com/D16373