Free up slugs to be used on delete

Found out that the deleted slugs in the db must have been manually created, as friendly id does not do this.

This adds a after delete callback to concatenate the slug with `deleted-` so that the slug is then free to use.

This should prevent crazy --n numbers when editions have been deleted but then new editions with the same title have a slug clash.

https://www.pivotaltracker.com/story/show/41021231
https://www.pivotaltracker.com/story/show/49488553