Add administrative webhooks (#18510)

* Add administrative webhooks

* Fix error when webhook is deleted before delivery worker runs

app/policies/webhook_policy.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+class WebhookPolicy < ApplicationPolicy
+  def index?
+    admin?
+  end
+
+  def create?
+    admin?
+  end
+
+  def show?
+    admin?
+  end
+
+  def update?
+    admin?
+  end
+
+  def enable?
+    admin?
+  end
+
+  def disable?
+    admin?
+  end
+
+  def rotate_secret?
+    admin?
+  end
+
+  def destroy?
+    admin?
+  end
+end