RRS: add missing migration

This should have been added in 2c8f979f9c
to make it possible to delete an account that is the administrator for a
maintenance plan without removing the maintenance plan.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

rrs/migrations/0019_maintplan_admin_delete_null.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2019-02-25 22:50
+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 = [
+        ('rrs', '0018_rmh_unique'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='maintenanceplan',
+            name='admin',
+            field=models.ForeignKey(blank=True, help_text='Plan administrator', null=True, on_delete=django.db.models.deletion.SET_NULL, to=settings.AUTH_USER_MODEL),
+        ),
+    ]