refactor: migrate ENABLE_MAX_FAILED_LOGIN_ATTEMPTS off FEATURES-as-dict

Already a flat setting in both lms/envs/common.py (True) and
cms/envs/common.py (False); switch the subscript reader in shared student
user models to settings.X and convert the six test overrides (patch.dict
and whole-dict override_settings(FEATURES=...) forms) to flat overrides.