UI - add ability to customize header logo

Summary: Fixes T7165. Let users specify a file phid in config, and then use that file via an inline style tag. Also, cache the URI so that we don't have to query the file on every page load.

Test Plan:

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7165

Differential Revision: https://secure.phabricator.com/D11886