Fix quantity sent as string when saving an asset

submitAssetForm() converted item_type_id/folder_id/status_id/
assigned_user_id from FormData's string values to numbers before
JSON-encoding the payload, but missed quantity, so it stayed a string
and the server's json.Decode into assetRequest.Quantity (int) rejected
it as "cannot unmarshal string into Go struct field".

web/static/js/app.js

@@ -713,6 +713,8 @@
         payload.status_id = Number(value);
       } else if (key === 'assigned_user_id') {
         payload.assigned_user_id = value ? Number(value) : null;
+      } else if (key === 'quantity') {
+        payload.quantity = Number(value) || 0;
       } else {
         payload[key] = value;
       }