Make recurring period a database model

- For easier handling (foreignkeys, many2many)
- For higher flexibility (users can define their own periods)

uncloud/management/commands/db-add-defaults.py

@@ -0,0 +1,11 @@
+from django.core.management.base import BaseCommand
+from uncloud_pay.models import RecurringPeriod
+
+class Command(BaseCommand):
+    help = 'Add standard uncloud values'
+
+    def add_arguments(self, parser):
+        pass
+
+    def handle(self, *args, **options):
+        RecurringPeriod.populate_db_defaults()