Rename Part.URL -> part.link

- Previous migration needed some tweaking to get it to run... because.. why?
- It seems to pass when running manage.py migrate but fails when running manage.py test
- Stumped on this one

InvenTree/part/forms.py

@@ -120,7 +120,7 @@ class EditPartForm(HelperForm):
             'keywords',
             'variant_of',
             'is_template',
-            'URL',
+            'link',
             'default_location',
             'default_supplier',
             'units',