Add administrative webhooks (#18510)

* Add administrative webhooks

* Fix error when webhook is deleted before delivery worker runs

app/workers/webhooks/delivery_worker.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+class Webhooks::DeliveryWorker
+  include Sidekiq::Worker
+  include JsonLdHelper
+
+  sidekiq_options queue: 'push', retry: 16, dead: false
+
+  def perform(webhook_id, body)
+    @webhook   = Webhook.find(webhook_id)
+    @body      = body
+    @response  = nil
+
+    perform_request
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+
+  private
+
+  def perform_request
+    request = Request.new(:post, @webhook.url, body: @body)
+
+    request.add_headers(
+      'Content-Type' => 'application/json',
+      'X-Hub-Signature' => "sha256=#{signature}"
+    )
+
+    request.perform do |response|
+      raise Mastodon::UnexpectedResponseError, response unless response_successful?(response) || response_error_unsalvageable?(response)
+    end
+  end
+
+  def signature
+    OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), @webhook.secret, @body)
+  end
+end