Add administrative webhooks (#18510)

* Add administrative webhooks

* Fix error when webhook is deleted before delivery worker runs

db/migrate/20220606044941_create_webhooks.rb

@@ -0,0 +1,12 @@
+class CreateWebhooks < ActiveRecord::Migration[6.1]
+  def change
+    create_table :webhooks do |t|
+      t.string :url, null: false, index: { unique: true }
+      t.string :events, array: true, null: false, default: []
+      t.string :secret, null: false, default: ''
+      t.boolean :enabled, null: false, default: true
+
+      t.timestamps
+    end
+  end
+end