billing: Move exempt_from_from_license_number_check to Customer model.

This belongs more on the Customer model, since this is a similar
attribute to default_discount.

corporate/lib/registration.py

@@ -72,7 +72,11 @@ def check_spare_licenses_available_for_adding_new_users(
     realm: Realm, number_of_users_to_add: int
 ) -> None:
     plan = get_current_plan_by_realm(realm)
-    if plan is None or plan.automanage_licenses or plan.exempt_from_from_license_number_check:
+    if (
+        plan is None
+        or plan.automanage_licenses
+        or plan.customer.exempt_from_from_license_number_check
+    ):
         return
 
     if plan.licenses() < get_latest_seat_count(realm) + number_of_users_to_add:

corporate/migrations/0011_move_exempt_from_from_license_number_check_to_customer_model.py

@@ -0,0 +1,27 @@
+# Generated by Django 3.2.4 on 2021-06-18 18:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    """
+    We haven't set the values for this field for the relevant organizations
+    as of this moment, so we can simply drop the column from CustomerPlan
+    and add it to Customer without worrying about losing the values.
+    """
+
+    dependencies = [
+        ("corporate", "0010_customerplan_exempt_from_from_license_number_check"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="customerplan",
+            name="exempt_from_from_license_number_check",
+        ),
+        migrations.AddField(
+            model_name="customer",
+            name="exempt_from_from_license_number_check",
+            field=models.BooleanField(default=False),
+        ),
+    ]

corporate/models.py

@@ -22,6 +22,9 @@ class Customer(models.Model):
     default_discount: Optional[Decimal] = models.DecimalField(
         decimal_places=4, max_digits=7, null=True
     )
+    # Some non-profit organizations on manual license management pay only for their paid employees.
+    # We don't prevent these organizations from adding more users than the number of licenses they purchased.
+    exempt_from_from_license_number_check: bool = models.BooleanField(default=False)
 
     def __str__(self) -> str:
         return f"<Customer {self.realm} {self.stripe_customer_id}>"
@@ -45,10 +48,6 @@ class CustomerPlan(models.Model):
     automanage_licenses: bool = models.BooleanField(default=False)
     charge_automatically: bool = models.BooleanField(default=False)
 
-    # Some non-profit organizations on manual license management pay only for their paid employees.
-    # We don't prevent these organizations from adding more users than the number of licenses they purchased.
-    exempt_from_from_license_number_check: bool = models.BooleanField(default=False)
-
     # Both of these are in cents. Exactly one of price_per_license or
     # fixed_price should be set. fixed_price is only for manual deals, and
     # can't be set via the self-serve billing system.