Code review feedback and test cleanup.

common/djangoapps/student/models.py

common/djangoapps/student/tests/test_events.py

common/djangoapps/student/tests/tests.py

common/djangoapps/util/model_utils.py

common/test/acceptance/pages/lms/learner_profile.py

common/test/acceptance/tests/helpers.py

common/test/acceptance/tests/lms/test_account_settings.py

common/test/acceptance/tests/lms/test_learner_profile.py

openedx/core/djangoapps/user_api/accounts/api.py

openedx/core/djangoapps/user_api/accounts/tests/test_api.py

openedx/core/djangoapps/user_api/models.py

openedx/core/djangoapps/user_api/tests/test_models.py