Move ui.custom_header to UI Config

Summary: Moves the setting from Core to UI, also adds a link to the task for further instructions.

Test Plan: Load up config in sandbox, see new instructions.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4214

Differential Revision: https://secure.phabricator.com/D11900