Invaluable information:
https://bobcares.com/blog/permission-denied-for-database-postgres/

https://stackoverflow.com/questions/14186055/django-test-app-error-got-an-error-creating-the-test-database-permission-deni

And this seems to be necessary (if 'admin' is the database username in settings.py):
ideatree=# GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA public to admin;
GRANT
ideatree=# GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA public to admin;
GRANT
ideatree=# GRANT ALL PRIVILEGES ON ALL FUNCTIONS IN SCHEMA public to admin;
GRANT