Add 'serial_int' field to stockitem model

- Allows for integer sorting of stock items

InvenTree/stock/migrations/0068_stockitem_serial_int.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.5 on 2021-11-09 23:30
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0067_alter_stockitem_part'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='serial_int',
+            field=models.IntegerField(default=0),
+        ),
+    ]

InvenTree/stock/models.py

@@ -504,6 +504,8 @@ class StockItem(MPTTModel):
         help_text=_('Serial number for this item')
     )
 
+    serial_int = models.IntegerField(default=0)
+
     link = InvenTreeURLField(
         verbose_name=_('External Link'),
         max_length=125, blank=True,