Correctly handle the update_fields kwarg on Category.save()

If the update_fields is given as kwarg and doesn’t contain the slug as
field to update then skip validating uniqueness of the slug and updating
the children categories.

I also think that we can deprecate the update_slugs kwarg from the save method and move it within the added condition. 
Also for conciseness I'd use `update_fields = kwargs.get('update_fields', [])` and the only `if 'slug' in update_fields`