Resolve "Changing feedback that a reviewer worken on is forbidden after set to final"

Closes #168 (closed)

core/migrations/0018_auto_20190709_1526.py

+# Generated by Django 2.2 on 2019-07-09 15:26
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0017_auto_20190604_1631'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='feedback',
+            name='final_by_reviewer',
+            field=models.BooleanField(default=False),
+        ),
+        migrations.AlterField(
+            model_name='feedbacklabel',
+            name='name',
+            field=models.CharField(max_length=50, unique=True),
+        ),
+    ]