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/part/migrations/0014_auto_20190502_2039.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2 on 2019-05-02 10:39
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0013_auto_20190429_2229'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='part',
+            name='URL',
+            field=models.URLField(blank=True, help_text='Link to extenal URL'),
+        ),
+    ]