Allow custom request checker prior to displaying page

Summary: We want to allow a broader access to our installation but we need to check the request in that case.

Test Plan:
Created a simple `PhabricatorRequestChecker` returning a custom controller.
Verified that this controller is used when accessing any page.
Returned `null` from this checker and verified that all 209 Phabricator pages are accessible.

Reviewers: epriestley

Reviewed By: epriestley

CC: scottmac, aran, Korvin, btrahan

Differential Revision: https://secure.phabricator.com/D2488

src/applications/base/controller/PhabricatorController.php

@@ -79,6 +79,19 @@ abstract class PhabricatorController extends AphrontController {
       return $this->delegateToController($disabled_user_controller);
     }
 
+    $event = new PhabricatorEvent(
+      PhabricatorEventType::TYPE_CONTROLLER_CHECKREQUEST,
+      array(
+        'request' => $request,
+        'controller' => get_class($this),
+      ));
+    $event->setUser($user);
+    PhutilEventEngine::dispatchEvent($event);
+    $checker_controller = $event->getValue('controller');
+    if ($checker_controller != get_class($this)) {
+      return $this->delegateToController($checker_controller);
+    }
+
     if (PhabricatorEnv::getEnvConfig('darkconsole.enabled')) {
       if ($user->getConsoleEnabled() ||
           PhabricatorEnv::getEnvConfig('darkconsole.always-on')) {