billing: Downgrade small realms that are behind on payments.

An organization with at most 5 users that is behind on payments isn't
worth spending time on investigating the situation.

For larger organizations, we likely want somewhat different logic that
at least does not void invoices.

zilencer/management/commands/downgrade_small_realms_behind_on_payments.py

@@ -0,0 +1,11 @@
+from typing import Any
+
+from corporate.lib.stripe import downgrade_small_realms_behind_on_payments_as_needed
+from zerver.lib.management import ZulipBaseCommand
+
+
+class Command(ZulipBaseCommand):
+    help = "Downgrade small realms that are running behind on payments"
+
+    def handle(self, *args: Any, **options: Any) -> None:
+        downgrade_small_realms_behind_on_payments_as_needed()