🐛(aliases) alias destination can be devnull@devnull

devnull@devnull is not considered a valid email address by django's
EmailFieldValidator but it's a special address in dimail's config.

Make "destination" a CharField instead of an EmailField to replace
validator and add devnull to allowlist.

src/backend/mailbox_manager/tests/models/test_aliases.py

@@ -0,0 +1,19 @@
+"""
+Unit tests for the Alias model
+"""
+
+import pytest
+
+from mailbox_manager import factories, models
+
+pytestmark = pytest.mark.django_db
+
+
+def test_models_aliases__devnull_destination_ok():
+    """Can create alias where destination is devnull@devnull."""
+
+    models.Alias.objects.create(
+        local_part="spam",
+        domain=factories.MailDomainEnabledFactory(),
+        destination="devnull@devnull",
+    )