Move coupon code routes out of order resources

Goal
----

As a SolidusStarterFrontend user, I want the coupon code routes moved
out of the order resource routes because the coupon codes controller
already depends on the current order. As such, the coupon code routes do
not need to maintain an order parameter.

template.rb

@@ -172,9 +172,9 @@
 
     get '/orders/:id/token/:token' => 'orders#show', as: :token_order
 
-    resources :orders, only: :show do
-      resources :coupon_codes, only: :create
-    end
+    resources :orders, only: :show
+
+    resources :coupon_codes, only: :create
 
     resource :cart, only: [:edit, :update] do
       put 'empty'

templates/app/views/orders/_coupon_code.html.erb

@@ -1,4 +1,4 @@
-<%= form_tag order_coupon_codes_path(@order), method: :post, class: "coupon-code" do %>
+<%= form_tag coupon_codes_path, method: :post, class: "coupon-code" do %>
   <div class="coupon-code__input">
     <div class="text-input text-input--small">
       <%= text_field_tag 'coupon_code',