Add a 'reference_int' field to the models, to be used as a secondary index

InvenTree/build/migrations/0031_build_reference_int.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.5 on 2021-10-14 06:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0030_alter_build_reference'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='reference_int',
+            field=models.IntegerField(default=0),
+        ),
+    ]