When a custom logo is configured but has a restrictive visibility policy, fall back to the default logo

Summary: Fixes T11982. Currently, configuring a custom logo and then setting the policy restrictively locks off the whole install.

Test Plan:

• Configured ui.logo.
• Searched for the file PHID in global search to find the underlying file.
• Set the policy to something restrictive ("only me").
• Purged cache (bin/cache purge --purge-all).
• Restarted webserver to nuke APC.
• Loaded a page as a different user.
• Before change: policy exception while trying to load the logo.
• After change: fallback to default logo.
• Loaded page as user who can see the logo, got custom logo.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11982

Differential Revision: https://secure.phabricator.com/D17011