Don't hack into AM to add mail previews

Instead just append our AM preview path let the app control wether
they're enabled or not, without forcing it only on some environments.

core/lib/spree/core/engine.rb

@@ -55,18 +55,16 @@ class Engine < ::Rails::Engine
         end
       end
 
-      config.after_initialize do
+      ActiveSupport.on_load(:action_mailer) do
         # Load in mailer previews for apps to use in development.
         # We need to make sure we call `Preview.all` before requiring our
         # previews, otherwise any previews the app attempts to add need to be
         # manually required.
-        if Rails.env.development? || Rails.env.test?
-          ActionMailer::Preview.all
 
-          Dir[root.join("lib/spree/mailer_previews/**/*_preview.rb")].each do |file|
-            require_dependency file
-          end
-        end
+        original_preview_path = preview_path
+        solidus_preview_path = Spree::Core::Engine.root.join 'lib/spree/mailer_previews/**/*_preview.rb'
+
+        self.preview_path = "{#{original_preview_path},#{solidus_preview_path}}"
       end
     end
   end