Make new logo and wordmark more reasonably configurable by human users
Summary: Fixes T11437. Provides a normal form for configuring this, instead of weird "look up the PHID and adjust things in the database" stuff.
Test Plan:
{F1753651}
{F1753652}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11437
Differential Revision: https://secure.phabricator.com/D16377