contest and contest_problems api.

add ordering for contest and submission models

utils/api/_serializers.py

@@ -4,7 +4,7 @@ from rest_framework import serializers
 
 class DateTimeTZField(serializers.DateTimeField):
     def to_representation(self, value):
-        # self.format = "%Y-%-m-%d %H:%M:%S %Z"
+        # self.format = "%Y-%m-%d %H:%M:%S %Z"
         value = timezone.localtime(value)
         return super(DateTimeTZField, self).to_representation(value)