🗃️(models) add MailDomain, MailDomainAccess and Mailbox models

Additional app and models to handle email addresses creation in Desk.

src/backend/mailbox_manager/tests/test_models_maildomain.py

@@ -0,0 +1,26 @@
+"""
+Unit tests for the MailDomain model
+"""
+
+from django.core.exceptions import ValidationError
+
+import pytest
+
+from mailbox_manager import factories
+
+pytestmark = pytest.mark.django_db
+
+
+# NAME FIELD
+
+
+def test_models_mail_domain__domain_name_should_not_be_empty():
+    """The domain name field should not be empty."""
+    with pytest.raises(ValidationError, match="This field cannot be blank"):
+        factories.MailDomainFactory(name="")
+
+
+def test_models_mail_domain__domain_name_should_not_be_null():
+    """The domain name field should not be null."""
+    with pytest.raises(ValidationError, match="This field cannot be null."):
+        factories.MailDomainFactory(name=None)