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. 
In general I think we have to get clear on what the semantics of these two calls is supposed to be:
* `self.save(update_fields=['slug'])`
* `self.save(update_fields=['name'])`