Add administrative webhooks (#18510)

* Add administrative webhooks

* Fix error when webhook is deleted before delivery worker runs

app/presenters/webhooks/event_presenter.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class Webhooks::EventPresenter < ActiveModelSerializers::Model
+  attributes :type, :created_at, :object
+
+  def initialize(type, object)
+    super()
+
+    @type       = type
+    @created_at = Time.now.utc
+    @object     = object
+  end
+end