Allow installs to add a custom corp/org header link
Summary: A bunch of installs are doing this to varying degrees of success anyway, make it easier and nudge them toward a more consistent approach.
Test Plan: Set a custom logo, viewed normal and admin pages.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T700
Differential Revision: https://secure.phabricator.com/D2019
