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 everything with a catchall rule if they don't want this behavior for some reason, so this should be strictly more powerful than the old behavior.)

See next diff for CORGI.

Test Plan:
  - Visited real 404 (like "/asdfafewfq"), missing-slash-404 (like "/maniphest") and real page (like "/maniphest/") URIs on blog, main, and CORGI sites.
  - Got 404 behavior, redirects, and real pages, respectively.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16966

src/aphront/AphrontRequest.php

@@ -545,6 +545,13 @@ final class AphrontRequest extends Phobject {
     return id(new PhutilURI($path))->setQueryParams($get);
   }
 
+  public function getAbsoluteRequestURI() {
+    $uri = $this->getRequestURI();
+    $uri->setDomain($this->getHost());
+    $uri->setProtocol($this->isHTTPS() ? 'https' : 'http');
+    return $uri;
+  }
+
   public function isDialogFormPost() {
     return $this->isFormPost() && $this->getStr('__dialog__');
   }