Pay: move some model-related methods from helpers to models

Otherwise we end up in circular dependency hell

uncloud/uncloud_pay/management/commands/generate-bills.py

@@ -7,7 +7,7 @@ from django.core.exceptions import ObjectDoesNotExist
 
 from datetime import timedelta, date
 from django.utils import timezone
-from uncloud_pay.helpers import generate_bills_for
+from uncloud_pay.models import Bill
 
 BILL_PAYMENT_DELAY=timedelta(days=10)
 
@@ -28,7 +28,7 @@ class Command(BaseCommand):
 
         for user in users:
             now = timezone.now()
-            generate_bills_for(
+            Bill.generate_for(
                     year=now.year,
                     month=now.month,
                     user=user,