add SplitCheckoutController and conditional route

app/constraints/split_checkout_constraint.rb

@@ -0,0 +1,9 @@
+class SplitCheckoutConstraint
+  def matches?(request)
+    Flipper.enabled? :split_checkout, current_user(request)
+  end
+
+  def current_user(request)
+    @spree_current_user ||= request.env['warden'].user
+  end
+end

app/controllers/split_checkout_controller.rb

@@ -0,0 +1,3 @@
+class SplitCheckoutController < CheckoutController
+
+end

config/routes.rb

@@ -69,10 +69,17 @@ Openfoodnetwork::Application.routes.draw do
     resources :webhooks, only: [:create]
   end
 
-  get '/checkout', to: 'checkout#edit' , as: :checkout
-  put '/checkout', to: 'checkout#update' , as: :update_checkout
-  get '/checkout/:state', to: 'checkout#edit', as: :checkout_state
-  get '/checkout/paypal_payment/:order_id', to: 'checkout#paypal_payment', as: :paypal_payment
+  constraints(SplitCheckoutConstraint.new) do
+    get '/checkout', to: 'split_checkout#edit', as: :checkout
+    put '/checkout', to: 'split_checkout#update', as: :update_checkout
+    get '/checkout/:state', to: 'split_checkout#edit', as: :checkout_state
+    get '/checkout/paypal_payment/:order_id', to: 'split_checkout#paypal_payment', as: :paypal_payment
+  end
+
+  get '/checkout', to: 'checkout#edit'
+  put '/checkout', to: 'checkout#update'
+  get '/checkout/:state', to: 'checkout#edit'
+  get '/checkout/paypal_payment/:order_id', to: 'checkout#paypal_payment'
 
   get 'embedded_shopfront/shopfront_session', to: 'application#shopfront_session'
   post 'embedded_shopfront/enable', to: 'application#enable_embedded_styles'