Fix API inconsistencies for translation support

The whole app now operates on the premise that the parent object defines
the language code. No parent object == don't filter on the language.

This avoids various inconsistencies with the API's and behavior,
such as:
- where to allow a `language_code` parameter, or not.
- name `limit_language` vs `limit_parent_language`.
- no parent == filter on default language, current language, no language?
- how to avoid hurting non-multilingual sites

Changes:
- Renamed `limit_language` to `limit_parent_language` everywhere.
  Set the default to `True` now in all cases. This is safe and consistent.
- Renamed `qs.language()` to `qs.translated()` for consistency with django-parler.
Fix `render_placeholder` behavior, filters by language now.

- Made sure `render_placeholder` passes a parent object.
- Made sure `placeholder.get_content_items()` always filters stale objects.
- Optimize access to `object.placeholder.parent`, by filling the caches.
Include the language_code in the ContentItem delete representation

This code can be displayed in the admin delete configuration
APIDOCS: mention to effect of adding a ContentItemRelation to your model