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, given that `slug` is generated from `name` in `update_slug`, shouldn't the uniqueness check run also if `'name' in update_fields`?