text: Title: CMSPageRenderer shall add data to context only once
  Body: Currently the method `shop.rest.renderers.CMSPageRenderer.render` merges the serialized data, once into the `template_context` and once as extra attribute `template_context['data']`.
  
  This is confusing and may cause errors by polluting the global context namespace. Instead the serialized data shall only be added to `template_context['data']`.
  
  Additionally, the View class using that renderer shall be allowed to override the attribute name `data`. I would propose to add `context_data_name = 'my_data'` to the View class (in analogy to `context_object_name` in Django View classes).