修复公告列表中用户权限判断错误的情况，抽取代码为 decorator。

announcement/views.py

@@ -10,27 +10,13 @@ from group.models import Group
 from .models import Announcement
 from .serializers import (CreateAnnouncementSerializer, AnnouncementSerializer,
                           EditAnnouncementSerializer)
+from .decorators import check_user_announcement_permission
 
 
+@check_user_announcement_permission
 def announcement_page(request, announcement_id):
-    try:
-        announcement = Announcement.objects.get(id=announcement_id, visible=True)
-    except Announcement.DoesNotExist:
-        return error_page(request, u"公告不存在")
-    # 公开的公告
-    if announcement.is_global == 0:
-        return render(request, "oj/announcement/announcement.html", {"announcement": announcement})
-    else:
-        if not request.user.is_authenticated():
-            return error_page(request, u"公告不存在")
-        # 判断是不是在组里面
-        if request.user.admin_type == SUPER_ADMIN or request.user == announcement.created_by:
-            return render(request, "oj/announcement/announcement.html", {"announcement": announcement})
-        else:
-            if request.user.groups.filter(id__in=[item.id for item in announcement.groups.all()]).exists():
-                return render(request, "oj/announcement/announcement.html", {"announcement": announcement})
-            else:
-                return error_page(request, u"公告不存在")
+    announcement = Announcement.objects.get(id=announcement_id, visible=True)
+    return render(request, "oj/announcement/announcement.html", {"announcement": announcement})
 
 
 class AnnouncementAdminAPIView(APIView):