Mysociety-django-sluggable

* Update from upstream for current Django compatibility.

* Update from upstream for Django 1.8 compatibility.
* Deal with import move of GenericRelation.

* Default to a slugify.slugify function if present, to provide
better i18n support.

* Better Python 3 compatibility

* Add unique_with to restrict slug uniqueness to the fields specified
* Still generate uniqueness lookups when a uniqueness field is empty

* Current upstream verson