Add a 'uid' field to StockItem model

- To be used for barcode asociation and lookup

InvenTree/stock/migrations/0026_stockitem_uid.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.10 on 2020-04-14 12:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0025_auto_20200405_2243'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='uid',
+            field=models.CharField(blank=True, help_text='Unique identifier field', max_length=128),
+        ),
+    ]