Add index action for Images to API fixes spree/spree#5081

#5081 

- Added index action to Images controller
- Added rabl view for images index 
- Tests for both products and variants

In the Index action, if the scope is nil (either `product_id` or `variant_id` is not passed as a param), the index action fails 422.  Not sure if we should raise a missing parameter error?  I couldn't find any other examples in that API that do that but let me know. Thanks!