* Adds a Part API filter to limit query to valid conversion options for the specified part
* Refactor 'exclude_tree' filter to use django-filter framework
* Refactor the 'ancestor' filter
* Refactoring more API filtering fields:
- variant_of
- in_bom_for
* Adds API endpoint / view / serializer for converting a StockItem to variant
* stock item conversion now perfomed via the API
* Bump API version
* Add unit tests for new filtering option on the Part list API endpoint
* Adds unit test for "convert" API endpoint functionality