scripts: Added competitions/list_competitions.py
scripts: Added --only-running to competitions/list_competitions.py
scripts: Added --only-future/past to competitions/list_competitions.py
scripts: Added competitions/add_class.py
scripts: Added competitions/list_classes.py
scripts: Added competitions/delete_classes.py
scripts: Added --all to competitions/delete_classes.py
scripts: Renamed competitions/add_class.py to add_classes.py

.. and make sure that the script can handle creating multiple
competition classes at once.
scripts: Renamed comp./delete_participant.py to delete_participants.py

.. and make sure that the script can handle deleting multiple
competition participants at once.
scripts: Fixed help text in competitions/delete_classes.py
scripts: Added --all to competitions/delete_participants.py
scripts: Improved --all checks in competition scripts