Add quantity field to StockItemTracking

- Keep track of historical levels of part availability

InvenTree/stock/models.py

@@ -209,6 +209,7 @@ class StockItem(models.Model):
             item=self,
             title=title,
             user=user,
+            quantity=self.quantity,
             date=datetime.now().date(),
             notes=notes,
             system=system
@@ -346,6 +347,9 @@ class StockItemTracking(models.Model):
     # Was this tracking note auto-generated by the system?
     system = models.BooleanField(default=False)
 
+    # Keep track of the StockItem quantity throughout the tracking history
+    quantity = models.PositiveIntegerField(validators=[MinValueValidator(0)], default=1)
+
     # TODO
     # image = models.ImageField(upload_to=func, max_length=255, null=True, blank=True)