Fix order_path issue in cart page

The order_path route wasn't being inferred correctly from this Spree controller, causing errors to be thrown in the cart page if the number of editable orders was precisely one:

ActionView::Template::Error: undefined method `order_path'

app/helpers/spree/orders_helper.rb

@@ -30,7 +30,7 @@ module Spree
     end
 
     def changeable_orders_link_path
-      changeable_orders.one? ? order_path(changeable_orders.first) : spree.account_path
+      changeable_orders.one? ? main_app.order_path(changeable_orders.first) : spree.account_path
     end
 
     def shop_changeable_orders_alert_html

spec/features/consumer/shopping/cart_spec.rb

@@ -315,6 +315,16 @@ feature "full-page cart", js: true do
         expect(page).to have_no_content item1.variant.name
         expect(page).to have_content item2.variant.name
       end
+
+      context "with a single editable order" do # Regression test for #8191
+        before do
+          prev_order2.destroy
+        end
+
+        it "doesn't throw an error" do
+          expect{ visit main_app.cart_path }.to_not raise_error
+        end
+      end
     end
   end
 end