calendar utils: Fix parsing and deleting old calandars

calendar_utils/management/commands/update_callendars.py

calendar_utils/models.py

calendar_utils/parser.py

tests/calendar_utils/conftest.py

tests/calendar_utils/samples/future_events.json

tests/calendar_utils/samples/past_events.json

tests/calendar_utils/samples/response.json

tests/calendar_utils/test_parser.py