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