Add support for primary address in user.

Closes #35

Fixes #35

uncloud_auth/models.py

@@ -3,7 +3,6 @@ from django.db import models
 from django.core.validators import MinValueValidator
 
 from uncloud_pay import AMOUNT_DECIMALS, AMOUNT_MAX_DIGITS
-
 from uncloud_pay.models import get_balance_for_user
 
 class User(AbstractUser):
@@ -18,6 +17,12 @@ class User(AbstractUser):
             decimal_places=AMOUNT_DECIMALS,
             validators=[MinValueValidator(0)])
 
+    # Need to use the string here to prevent a circular import
+    primary_billing_address = models.ForeignKey('uncloud_pay.BillingAddress',
+                                                   on_delete=models.PROTECT,
+                                                   blank=True,
+                                                   null=True)
+
     @property
     def balance(self):
         return get_balance_for_user(self)