Add (optional) URL to stock tracking info

InvenTree/stock/models.py

@@ -512,6 +512,7 @@ class StockItemTracking(models.Model):
         date: Date that this tracking info was created
         title: Title of this tracking info (generated by system)
         notes: Associated notes (input by user)
+        URL: Optional URL to external page
         user: The user associated with this tracking info
         quantity: The StockItem quantity at this point in time
     """
@@ -529,6 +530,8 @@ class StockItemTracking(models.Model):
 
     notes = models.TextField(blank=True)
 
+    URL = models.URLField(blank=True, help_text='Link to external page for further information')
+
     user = models.ForeignKey(User, on_delete=models.SET_NULL, blank=True, null=True)
 
     system = models.BooleanField(default=False)