stripe: Set min license count based on flat discount.

For self hosted basic plan, we need to allow customers to subscribe
without purchasing 10 licenses and also we need to allow customer
take fully use the available discount so that if the add more
users in the future, the full discount was already applied.

To fix above, we set minimum user count to the least number
of licenses we require for the charge to be positive after applying
the complete discount.

corporate/lib/stripe.py

@@ -2069,6 +2069,23 @@ class BillingSession(ABC):
 
         return None, context
 
+    def min_licenses_for_flat_discount_to_self_hosted_basic_plan(
+        self, customer: Optional[Customer]
+    ) -> int:
+        # Since monthly and annual TIER_SELF_HOSTED_BASIC plans have same per user price we only need to do this calculation once.
+        # If we decided to apply this for other tiers, then we will have to do this calculation based on billing schedule selected by the user.
+        price_per_license = get_price_per_license(
+            CustomerPlan.TIER_SELF_HOSTED_BASIC, CustomerPlan.BILLING_SCHEDULE_MONTHLY
+        )
+        if customer is None:
+            return (
+                Customer._meta.get_field("flat_discount").get_default() // price_per_license
+            ) + 1
+        elif customer.flat_discounted_months > 0:
+            return (customer.flat_discount // price_per_license) + 1
+        # If flat discount is not applied.
+        return 1
+
     def min_licenses_for_plan(self, tier: int) -> int:
         customer = self.get_customer()
         if customer is not None and customer.minimum_licenses:
@@ -2076,7 +2093,7 @@ class BillingSession(ABC):
             return customer.minimum_licenses
 
         if tier == CustomerPlan.TIER_SELF_HOSTED_BASIC:
-            return 10
+            return min(self.min_licenses_for_flat_discount_to_self_hosted_basic_plan(customer), 10)
         if tier == CustomerPlan.TIER_SELF_HOSTED_BUSINESS:
             return 25
         return 1