billing: Add option to request a sponsorship in /upgrade.

corporate/models.py

@@ -11,6 +11,7 @@ from zerver.models import Realm
 class Customer(models.Model):
     realm: Realm = models.OneToOneField(Realm, on_delete=CASCADE)
     stripe_customer_id: str = models.CharField(max_length=255, null=True, unique=True)
+    sponsorship_pending: bool = models.BooleanField(default=False)
     # A percentage, like 85.
     default_discount: Optional[Decimal] = models.DecimalField(decimal_places=4, max_digits=7, null=True)