Major major major (improvements for StockItem list API)

OK LISTEN UP - Lots of work went into making this speedier:

- For related detail fields (e.g. part_detail), we pre-fetch and cache the model data
- This eliminates duplicate database hits for the same model instances
- Perform all field filtering manually, rather than using the DRF 'filter_fields' concept (this seems to add a lot of overhead)
- Use query annotations to getch calculated fields rather than fetching one-at-a-time
- And finally, if the request is AJAX then return a JsonResponse which is SO FREAKING MUCH FASTER

InvenTree/part/api.py

@@ -293,10 +293,10 @@ class PartList(generics.ListCreateAPIView):
 
     def filter_queryset(self, queryset):
         """
-        Perform custom filtering of the queryset
+        Perform custom filtering of the queryset.
+        We overide the DRF filter_fields here because 
         """
 
-        # Perform basic filtering
         queryset = super().filter_queryset(queryset)
 
         # Filter by 'starred' parts?