add signals to mails, improve and update tests

oser_backend/mails/apps.py

oser_backend/mails/notifications.py

oser_backend/mails/signals.py

oser_backend/mails/tests.py

oser_backend/oser_backend/settings/dev.py

oser_backend/tests/test_visits/test_abandon.py

oser_backend/tests/test_visits/test_participation_api.py

oser_backend/tests/test_visits/test_signals.py

oser_backend/tests/utils/mixins.py

oser_backend/visits/apps.py

oser_backend/visits/factory.py

oser_backend/visits/models.py

oser_backend/visits/notifications.py

oser_backend/visits/serializers.py

oser_backend/visits/signals.py