add email notification when changing project participation state

oser_backend/settings/common.py

profiles/factory.py

projects/models.py

projects/notifications.py

projects/signals.py

.../templates/projects/organizers_participation_cancelled.md

...ts/templates/projects/organizers_participation_deleted.md

...s/templates/projects/organizers_participation_received.md

projects/templates/projects/participation_accepted.md

projects/templates/projects/participation_cancelled.md

projects/templates/projects/participation_deleted.md

projects/templates/projects/participation_received.md

projects/templates/projects/participation_rejected.md

projects/templates/projects/participation_valid.md

projects/templates/projects/to_user.md

tests/test_projects/test_signals.py

tests/test_visits/test_signals.py

tests/utils/mixins.py

visits/models.py