R1:36c6eb966393

Improve routing of "/robots.txt", "/favicon.ico", "/status/", and 404 on custom Sites

Summary:
Fixes T12919. Fixes T13636. Prior to this change, some well-known resource paths don't route on sites like ResourceSite.

- `/robots.txt`: Make it route on ResourceSite and just deny the whole site.
- `/favicon.ico`: Make it route on ResourceSite.
- `/status/`: Make it route on ResourceSite.
- 404: Make it render a 404 on ResourceSite.

Test Plan:
- Visited all URIs on ResourceSite, got sensible responses.