Add support for primary address in user.

Closes #35

Fixes #35

uncloud_pay/migrations/0014_paymentsettings.py

@@ -0,0 +1,25 @@
+# Generated by Django 3.0.6 on 2020-05-10 13:53
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+import uuid
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('uncloud_pay', '0013_auto_20200508_1446'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='PaymentSettings',
+            fields=[
+                ('uuid', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('owner', models.OneToOneField(editable=False, on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
+                ('primary_billing_address', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='uncloud_pay.BillingAddress')),
+            ],
+        ),
+    ]