Fix url and admin check in contest (#67)

- Fix url
 - Add contest_announcement get and fix bug

contest/urls/oj.py

@@ -3,5 +3,5 @@ from django.conf.urls import url
 from ..views.oj import ContestAnnouncementListAPI
 
 urlpatterns = [
-    url(r"^contest", ContestAnnouncementListAPI.as_view(), name="contest_list_api"),
+    url(r"^contest$", ContestAnnouncementListAPI.as_view(), name="contest_list_api"),
 ]