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