Django 1.11 & some added fields and improvements

New in this version

• update to Django 1.11 (no changes required)
• use new "release" feature in Heroku Procfile for application of migrations
• add new fields to Order:
  • po_number, is_urgent
• add new fields to Product:
  • manufacturer, manufacturer_catalog
• fine-tuning of Order page and Product and Order tables