Add custom migration

- Required to initialize the MPTT fields for the StockItem model

InvenTree/stock/migrations/0022_auto_20200217_1109.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.9 on 2020-02-17 11:09
+
+from django.db import migrations
+from stock import models
+
+
+def update_stock_item_tree(apps, schema_editor):
+    # Update the StockItem MPTT model
+
+    models.StockItem.objects.rebuild()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0021_auto_20200215_2232'),
+    ]
+
+    operations = [
+        migrations.RunPython(update_stock_item_tree)
+    ]