Add in Django migration for shares.

problem_builder/migrations/0002_auto_20160121_1525.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+from django.conf import settings
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('problem_builder', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Share',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('submission_uid', models.CharField(max_length=32)),
+                ('block_id', models.CharField(max_length=255, db_index=True)),
+                ('notified', models.BooleanField(default=False, db_index=True)),
+                ('shared_by', models.ForeignKey(related_name='problem_builder_shared_by', to=settings.AUTH_USER_MODEL)),
+                ('shared_with', models.ForeignKey(related_name='problem_builder_shared_with', to=settings.AUTH_USER_MODEL)),
+            ],
+        ),
+        migrations.AlterUniqueTogether(
+            name='share',
+            unique_together=set([('shared_by', 'shared_with', 'block_id')]),
+        ),
+    ]