Add Supabase as backend provider (https://supabase.com/)

Supabase has received a lot of attention in recent years. It's open source alternative to firebase is now listed as one of the top repositories on GitHub. As many people are leaving Firebase (Google's implementation) it would be a great addition to iubenda in total. Supabase is a PostgreSQL SaaS free database-hosting alternative with a lot of features (including authentication and realtime databases). 

Thanks a lot for the very great tool :)
David