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/api.py

@@ -200,7 +200,7 @@ class PartList(generics.ListCreateAPIView):
             'description',
             'keywords',
             'is_template',
-            'URL',
+            'link',
             'units',
             'minimum_stock',
             'trackable',
@@ -249,12 +249,6 @@ class PartList(generics.ListCreateAPIView):
             else:
                 item['category__name'] = None
 
-            # Rename "URL" to "link" to distinguish from lower-case "url",
-            # which is the web address of the item itself
-            if 'URL' in item.keys():
-                item['link'] = item['URL']
-                del item['URL']
-
         return Response(data)
 
     def get_queryset(self):