When "phabricator.silent" is enabled, don't call webhooks

Summary: Ref T13078. The `phabricator.silent` configuration flag should disable webhook calls, since this is consistent with the documented and desired behavior.

Test Plan: Enabled `phabricator.silent`, made test hook calls, saw them fail with a "silent" failure reason.

Maniphest Tasks: T13078

Differential Revision: https://secure.phabricator.com/D19084

src/applications/herald/worker/HeraldWebhookWorker.php

@@ -32,6 +32,13 @@ final class HeraldWebhookWorker
           $status));
     }
 
+    // If we're in silent mode, permanently fail the webhook request and then
+    // return to complete this task.
+    if (PhabricatorEnv::getEnvConfig('phabricator.silent')) {
+      $this->failRequest($request, 'hook', 'silent');
+      return;
+    }
+
     $hook = $request->getWebhook();
 
     if ($hook->isDisabled()) {