Readd historical schedule fks (#23414)

* Readd historical schedule fks

openedx/core/djangoapps/schedules/migrations/0018_readd_historicalschedule_fks.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.28 on 2020-03-16 14:00
+from __future__ import unicode_literals
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('schedules', '0017_remove_start_from_historicalschedule'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='historicalschedule',
+            name='history_user',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+', to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.AlterField(
+            model_name='historicalschedule',
+            name='start_date',
+            field=models.DateTimeField(db_index=True, default=None, help_text='Date this schedule went into effect', null=True),
+        ),
+    ]