👔(dimail) allow creation of "pending" mailboxes

Previously, mailbox creation was restricted to "enabled" domains.
We now allow users to create mailboxes on pending and failed domains.
Mailboxes thus created have the "pending" mailboxes status.

src/backend/mailbox_manager/tests/test_models_maildomain.py

@@ -37,6 +37,21 @@ def test_models_mail_domain__slug_inferred_from_name():
     assert domain.slug == slugify(name)
 
 
+# "STATUS" FIELD
+
+
+def test_models_mail_domain__status_should_not_be_empty():
+    """Status field should not be empty."""
+    with pytest.raises(ValidationError, match="This field cannot be blank"):
+        factories.MailDomainFactory(status="")
+
+
+def test_models_mail_domain__status_should_not_be_null():
+    """Status field should not be null."""
+    with pytest.raises(ValidationError, match="This field cannot be null."):
+        factories.MailDomainFactory(status=None)
+
+
 # get_abilities