Add migration to populate site_values in SiteConfigurationHistory

Right now the ORM is very unhappy about the JSONField `site_values`
in SiteConfigurationHistory containing non-JSON (empty strings).  We
cannot even write a data migration using the ORM to populate the field
because that causes a JSONDeserializationError.  Therefore, we must
bypass the ORM and populate the values with raw SQL.

DENG-18

openedx/core/djangoapps/site_configuration/migrations/0005_populate_siteconfig_history_site_values.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+forward_sql = """
+UPDATE
+    site_configuration_siteconfigurationhistory
+SET
+    site_values = '{}';
+"""
+
+reverse_sql = """
+UPDATE
+    site_configuration_siteconfigurationhistory
+SET
+    site_values = '';
+"""
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('site_configuration', '0004_add_site_values_field'),
+    ]
+
+    operations = [
+        migrations.RunSQL(forward_sql, reverse_sql=reverse_sql),
+    ]