✨(demo) add a "demo" app to facilitate testing/working on the project

We designed it to allow creating a huge number of objects fast using
bulk creation.

src/backend/demo/tests/test_commands_create_demo.py

@@ -0,0 +1,32 @@
+"""Test the `create_demo` management command"""
+from unittest import mock
+
+from django.core.management import call_command
+from django.test import override_settings
+
+import pytest
+
+from core import models
+
+from demo import defaults
+
+TEST_NB_OBJECTS = {
+    "users": 5,
+    "teams": 3,
+    "max_identities_per_user": 3,
+    "max_users_per_team": 5,
+}
+
+pytestmark = pytest.mark.django_db
+
+
+@override_settings(DEBUG=True)
+@mock.patch.dict(defaults.NB_OBJECTS, TEST_NB_OBJECTS)
+def test_commands_create_demo():
+    """The create_demo management command should create objects as expected."""
+    call_command("create_demo")
+
+    assert models.User.objects.count() == 5
+    assert models.Identity.objects.exists()
+    assert models.Team.objects.count() == 3
+    assert models.TeamAccess.objects.count() >= 3