Add default settings to let new users be subscribed on trial

- Add the default user to a subscription trial
- Update associated unit tests

src/database/migrations/0015_alter_subscription_user.py

@@ -0,0 +1,21 @@
+# Generated by Django 4.2.5 on 2023-11-11 05:39
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("database", "0014_alter_googleuser_picture"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="subscription",
+            name="user",
+            field=models.OneToOneField(
+                on_delete=django.db.models.deletion.CASCADE, related_name="subscription", to=settings.AUTH_USER_MODEL
+            ),
+        ),
+    ]