Only pass 'next' to context if variable is set in GET request
Only pass 'next' to context if variable is set in GET request

Otherwise django will try to redirect to 'None' or other non-existent
URL.

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