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