Update default wordmark and logo

Summary:
The current default wordmark is "Phabricator" which is trademarked and the
default logo is also copyright.

(This change was made by @speck directly in the deployed instnace, bringing it into master now).

Test Plan: should be fine.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, speck

Differential Revision: https://we.phorge.it/D25048