Purge old absolute_urls of a model when model instance is updated.

[anderser]

When using Varnish to cache your pages (for long) and updating a model instance so that the slug and by that the url changes, varnish will still cache the old version.

If you use Django redirects app (https://docs.djangoproject.com/en/1.3/ref/contrib/redirects/) you can easily store old versions of absolute_urls when updating a model.

This addition checks if Django redirects is installed, and if so, purges all the old urls of the model you justed saved, so that the user gets a proper redirect and not an old version of your page if they use the old url.