Add support for primary address in user.

Closes #35

Fixes #35

uncloud_auth/migrations/0004_user_primary_billing_address.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.6 on 2020-05-10 17:31
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('uncloud_pay', '0014_paymentsettings'),
+        ('uncloud_auth', '0003_auto_20200318_1345'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='primary_billing_address',
+            field=models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, to='uncloud_pay.BillingAddress'),
+        ),
+    ]