Add Railtie to delayed_job_active_record

Explicitly requiring ActiveRecord models in
`lib/delayed_job_active_record.rb` may result in losing custom
configurations that are set in config/initializers on Rails.

Let' use Railtie and ActiveSupport.on_load to set it up so that
Delayed::Backend::ActiveRecord::Job model will respect arbitrary
configs.

lib/delayed/backend/active_record/railtie.rb

@@ -0,0 +1,14 @@
+module Delayed
+  module Backend
+    module ActiveRecord
+      class Railtie < ::Rails::Railtie
+        initializer 'delayed_job_active_record' do |_app|
+          ActiveSupport.on_load(:active_record) do
+            require "delayed/backend/active_record"
+            Delayed::Worker.backend = :active_record
+          end
+        end
+      end
+    end
+  end
+end

lib/delayed_job_active_record.rb

@@ -1,5 +1,10 @@
-require "active_record"
 require "delayed_job"
-require "delayed/backend/active_record"
 
-Delayed::Worker.backend = :active_record
+if defined?(Rails)
+  require "delayed/backend/active_record/railtie"
+else
+  require "active_record"
+  require "delayed/backend/active_record"
+
+  Delayed::Worker.backend = :active_record
+end