R1:9730f5a34fd7

Allow custom Sites to have custom 404 controllers

Summary:
Currently, custom Sites must match `.*` or similar to handle 404's, since the fallback is always generic.

This locks them out of the "redirect to canonicalize to `path/` code", so they currently have a choice between a custom 404 page or automatic correction of `/`.

Instead, allow the 404 controller to be constructed explicitly. Sites can now customize 404 by implementing this method and not matching everything.

(Sites can still match…