Add initial generate-bills and charge-negative-balance uncloud-pay

commands

uncloud/uncloud_pay/helpers.py

@@ -0,0 +1,12 @@
+from functools import reduce
+from .models import Bill, Payment
+
+def sum_amounts(entries):
+        return reduce(lambda acc, entry: acc + entry.amount, entries, 0)
+
+
+def get_balance_for(user):
+        bills = sum_amounts(Bill.objects.filter(owner=user))
+        payments = sum_amounts(Payment.objects.filter(owner=user))
+        return payments - bills
+