* backend: automatically create super user, fixes#57
- if SUPERUSER_EMAIL is set, superuser is created with `is_superuser` and `is_verified` settings, if user doesn't already exist.
- if SUPERUSER_PASSWORD if set, the password for superuser is set, otherwise a random password is generated
update sample SUPERUSER_EMAIL and SUPERUSER_PASSWORD in config file and chart.
- ensure verification email is not sent if user already verified