Include custom header PHID in header logo cache key

Summary: Ref T4214. Reduce the need for installs to purge caches.

Test Plan:

• Changed header twice.
• New one showed up without dirtying cache.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4214

Differential Revision: https://secure.phabricator.com/D11965