R1:8640ab5fc3e4

Redirect `/source/x` (no slash) to `/source/x/` (canonical) when viewer is logged out and "x" is public

Summary:
Fixes T12035. Normally, the "abc" -> "abc/" redirect is handled automatically when "abc" hits a 404.

However, in this case, "source/x" does not 404. We route this to a valid controller because some VCS requests omit the slashes, then manually perform the redirect if we aren't serving a VCS request.

Allow this controller to serve public resources so we can serve the redirect to logged-out…