🗃️(database) create invitation model

Create invitation model, factory and related tests to prepare back-end
for invitation endpoints. We chose to use a separate dedicated model
for separation of concerns, see
https://github.com/numerique-gouv/people/issues/25

src/backend/core/tests/test_models_invitations.py

@@ -0,0 +1,46 @@
+"""
+Unit tests for the Invitation model
+"""
+from django.core.exceptions import ValidationError
+
+import pytest
+
+from core import factories
+
+pytestmark = pytest.mark.django_db
+
+
+def test_models_invitations_email_no_empty_mail():
+    """The "email" field should not be empty."""
+    with pytest.raises(ValidationError, match="This field cannot be blank"):
+        factories.InvitationFactory(email="")
+
+
+def test_models_invitations_email_no_null_mail():
+    """The "email" field is required."""
+    with pytest.raises(ValidationError, match="This field cannot be null"):
+        factories.InvitationFactory(email=None)
+
+
+def test_models_invitations_team_required():
+    """The "team" field is required."""
+    with pytest.raises(ValidationError, match="This field cannot be null"):
+        factories.InvitationFactory(team=None)
+
+
+def test_models_invitations_team_should_be_team_instance():
+    """The "team" field should be a team instance."""
+    with pytest.raises(ValueError, match='Invitation.team" must be a "Team" instance'):
+        factories.InvitationFactory(team="ee")
+
+
+def test_models_invitations_role_required():
+    """The "role" field is required."""
+    with pytest.raises(ValidationError, match="This field cannot be blank"):
+        factories.InvitationFactory(role="")
+
+
+def test_models_invitations_role_among_choices():
+    """The "role" field should be a valid choice."""
+    with pytest.raises(ValidationError, match="Value 'boss' is not a valid choice"):
+        factories.InvitationFactory(role="boss")