Move paypal_button_preference helper method

Ref. solidusio#251

Using `Spree::CheckoutController.helper` and
`Spree::OrdersController.helper` in `config/initializers/braintree.rb`
to include helper modules does not work: at `checkout/payement` step an
exception is raised

```
ActionView::Template::Error (undefined method `paypal_button_preference`)
```

This commit move `paypal_button_preferences` (introduced with [1]) in
the already present `SolidusPaypalBraintree::BraintreeCheckoutHelper`
module, and adds the same helper method to the
`Spree::OrdersController` (in the same way done here [2]) to enable
the button also in the cart page.

[1] solidusio#236
[2] solidusio@28d0de9

app/controllers/solidus_paypal_braintree/orders_controller_decorator.rb

@@ -0,0 +1,10 @@
+module SolidusPaypalBraintree
+  module OrdersControllerDecorator
+
+    def self.prepended(base)
+      base.helper ::SolidusPaypalBraintree::BraintreeCheckoutHelper
+    end
+
+    ::Spree::OrdersController.prepend(self)
+  end
+end

app/helpers/solidus_paypal_braintree/braintree_checkout_helper.rb

@@ -40,5 +40,8 @@ def braintree_3ds_options_for(order)
       }
     end
 
+    def paypal_button_preference(key, store:)
+      store.braintree_configuration.preferences[key]
+    end
   end
 end

config/initializers/braintree.rb

@@ -1,8 +1,3 @@
 if SolidusSupport.backend_available?
   Spree::Admin::PaymentsController.helper :braintree_admin
 end
-
-if SolidusSupport.frontend_available?
-  Spree::CheckoutController.helper :braintree_checkout
-  Spree::OrdersController.helper :braintree_checkout
-end