Add admin bill generation endpoint

uncloud_pay/serializers.py

@@ -72,6 +72,7 @@ class OrderSerializer(serializers.ModelSerializer):
         model = Order
         fields = ['uuid', 'owner', 'description', 'creation_date', 'starting_date', 'ending_date',
                 'bill', 'recurring_period', 'recurring_price', 'one_time_price', 'replaced_by']
+        read_only_fields = ['replaced_by']
 
 
 ###