修复contest announcement的一些问题

contest/migrations/0007_contestannouncement_visible.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.4 on 2017-11-06 09:02
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('contest', '0006_auto_20171011_1214'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='contestannouncement',
+            name='visible',
+            field=models.BooleanField(default=True),
+        ),
+    ]