Add quantity field to StockItemTracking

- Keep track of historical levels of part availability

InvenTree/stock/migrations/0005_stockitemtracking_quantity.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-04-12 14:09
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0004_auto_20190412_2030'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitemtracking',
+            name='quantity',
+            field=models.PositiveIntegerField(default=1, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]