Fix help menu links for folks with diviner uninstalled

Summary: move code to uninstallable help app rather than diviner. Fixes T4690.

Test Plan: uninstalled diviner, noted no links, then moved the code and suddenly helpful help links showed up once more.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4690

Differential Revision: https://secure.phabricator.com/D8638