Fix: Panic on login when username but no password stored in database (possible due to import from foreign contest)

Add database field 'active' for 'taskgroup'. Reset it before contest/task updating, so taskgroups can be removed from contests again. Fixes #12.

Fix: Set min_grade and max_grade when saving contest. Was forgotten when rebasing branch cleanup_db_connectors on top of min-max-grade

Add database field positionalnumber to taskgroup and order taskgroups accordingly in contest view & identify taskgroups with first task instead of name