Make the slug field for exercises optional
Make the slug field for exercises optional

This is needed because for names that are completely non-ASCII, the slug
field is empty (since it is URL-friendly)

kjagoo/wger_stark,petervanderdoes/wger,rolandgeider/wger,DeveloperMal/wger,kjagoo/wger_stark,petervanderdoes/wger,kjagoo/wger_stark,wger-project/wger,petervanderdoes/wger,kjagoo/wger_stark,DeveloperMal/wger,wger-project/wger,DeveloperMal/wger,wger-project/wger,petervanderdoes/wger,rolandgeider/wger,rolandgeider/wger,DeveloperMal/wger,rolandgeider/wger,wger-project/wger