Reintroduce and deprecate Order#deliver_order_confirmation_email

Was removed as part of #3081,
but not being private it's better to keep it around with a deprecation
for a while.

core/app/models/spree/order.rb

@@ -435,6 +435,13 @@ def finalize!
       touch :completed_at
 
       Spree::Event.fire 'order_finalized', order: self
+
+      if method(:deliver_order_confirmation_email).owner != self.class
+        Spree::Deprecation.warn \
+          "deliver_order_confirmation_email has been deprecated and moved to " \
+          "Spree::MailerSubscriber#order_finalized, please move there any customizations.",
+          caller(1)
+      end
     end
 
     def fulfill!
@@ -443,6 +450,16 @@ def fulfill!
       save!
     end
 
+    def deliver_order_confirmation_email
+      Spree::Deprecation.warn \
+        "deliver_order_confirmation_email has been deprecated and moved to " \
+        "Spree::MailerSubscriber#order_finalized.",
+        caller(1)
+
+      Spree::Config.order_mailer_class.confirm_email(order).deliver_later
+      order.update_column(:confirmation_delivered, true)
+    end
+
     # Helper methods for checkout steps
     def paid?
       %w(paid credit_owed).include?(payment_state)