Add new optional SIGNUP_DEFAULT_GROUP setting

arkindex/project/checks.py

arkindex/project/config.py

arkindex/project/settings.py

arkindex/project/tests/config_samples/defaults.yaml

arkindex/project/tests/config_samples/errors.yaml

arkindex/project/tests/config_samples/expected_errors.yaml

arkindex/project/tests/config_samples/override.yaml

arkindex/project/tests/test_checks.py

arkindex/users/api.py

arkindex/users/tests/test_registration.py