* Improve filtering options for StockItem list
- Make use of StockFilter introspection
* Remove outdated filter
* remove outdated "max_results" parameter
* Fix cascade issue for stocklist API
* Add relationship filters to the StockItemFilter
* Fix filtering by 'status' and 'allocated'
* Refactor 'customer' and 'expired' filters
* Cleanup
* Adds unit test for top-level stock location filtering