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.
• Visited all URIs on main site.
• Visited 404 while logged out, got login page.

Maniphest Tasks: T13636, T12919

Differential Revision: https://secure.phabricator.com/D21606