Add empty :new action to Spree::UsersController

Without this, specs fail with
```
AbstractController::ActionNotFound:
        The new action could not be found for the :authorize_actions
        callback on Spree::UsersController, but it is listed in the controller's
        :only option.
```

I guess this is because the views are not in the standard place, thus
impeding Rails' ability to dynamically generate empty actions if
matching views are present. Anyways: This is good, because it's more
explicit.

lib/controllers/frontend/spree/users_controller.rb

@@ -4,6 +4,8 @@ class Spree::UsersController < Spree::StoreController
   skip_before_action :set_current_order, only: :show, raise: false
   prepend_before_action :authorize_actions, only: :new
 
+  def new; end
+
   def show
     load_object
     @orders = @user.orders.complete.order('completed_at desc')