Removed UUID field from StockItem

- QR code is now created based on a JSON object
- This will make it extensible to other things such as StockLocation
- Created a helper function to make Barcode objects

InvenTree/stock/migrations/0013_remove_stockitem_uuid.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2 on 2019-05-02 10:39
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0012_auto_20190502_0058'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='stockitem',
+            name='uuid',
+        ),
+    ]