controllers/competitions: Fixed typo
scripts: Added --class option to competitions/add_participant.py
scripts: Added competition class output to list_participants.py
scripts: Renamed competitions/add_participant.py to add_participants.py

.. and make sure that the script can handle creating multiple
participants at once.
templates/competitions: Show competition classes in participants list
scripts: Added competitions/list_flights.py
scripts: Added --class option to competitions/list_flights.py