contest and contest_problems api.

add ordering for contest and submission models

problem/urls/oj.py

@@ -1,8 +1,9 @@
 from django.conf.urls import url
 
-from ..views.oj import ProblemTagAPI, ProblemAPI
+from ..views.oj import ProblemTagAPI, ProblemAPI, ContestProblemAPI
 
 urlpatterns = [
     url(r"^problem/tags/?$", ProblemTagAPI.as_view(), name="problem_tag_list_api"),
     url(r"^problems/?$", ProblemAPI.as_view(), name="problem_list_api"),
+    url(r"^contest_problems/?$", ContestProblemAPI.as_view(), name="contest_problem_api"),
 ]