Fix Django 1.10 error and deprecation warning for old `context_instance` argument.
Fix Django 1.10 error and deprecation warning for old `context_instance` argument.

This introduces a backwards compatibility issue when relying on custom
`context_processors` for the merge template that you've provided to
`{% page_placeholder "slot" template="..." %}`.

django-fluent/django-fluent-contents,edoburu/django-fluent-contents,django-fluent/django-fluent-contents,django-fluent/django-fluent-contents,edoburu/django-fluent-contents,edoburu/django-fluent-contents