Validate uniqueness for StockItems

- If the Part is a variant of a template, ensure that the serial numbers are unique across all instances of the template
- Prevent instantiation of a StockItem for a part which has variants

InvenTree/stock/migrations/0003_auto_20190525_2303.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-05-25 13:03
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0002_auto_20190525_2226'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='part',
+            field=models.ForeignKey(help_text='Base part', limit_choices_to={'active': True, 'has_variants': False}, on_delete=django.db.models.deletion.CASCADE, related_name='stock_items', to='part.Part'),
+        ),
+    ]