add submitted field on participation, update api, admin, tests and participation email

oser_backend/tests/test_visits/test_participation.py

oser_backend/visits/admin.py

oser_backend/visits/migrations/0019_auto_20180526_2034.py

oser_backend/visits/models.py

oser_backend/visits/serializers.py

oser_backend/visits/templates/visits/participation.md

oser_backend/visits/views.py