Show external build links in applications

Summary: Fixes T8659. This isn't explicitly documented but I'm going to wait for a bit until the "Harbormaster" doc splits into internal/external builds to add docs for it. There's other similar stuff coming soon anyway.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8659

Differential Revision: https://secure.phabricator.com/D13903