Summary:
Fixes T4610. Open to suggestions, etc., if there's anything I'm missing.
Also:
- Moves these "system" endpoints into a real application.
- Makes `isUnlisted()` work a little more consistently.
Test Plan: Accessed `/robots.txt`, `/status/` and `/debug/`.
Reviewers: chad, btrahan
Reviewed By: btrahan
Subscribers: aran, epriestley
Maniphest Tasks: T4610
Differential Revision: https://secure.phabricator.com/D8532
