* Allow part category table to be ordered by part count
* Add queryset annotation for part-category part-count
- Uses subquery to annotate the part-count for sub-categories
- Huge reduction in number of queries
* Update 'pathstring' property of PartCategory and StockLocation
- No longer a dynamically calculated value
- Constructed when the model is saved, and then written to the database
- Limited to 250 characters
* Data migration to re-construct pathstring for PartCategory objects
* Fix for tree model save() method
* Add unit tests for pathstring construction
* Data migration for StockLocation pathstring values
* Update part API
- Add new annotation to PartLocationDetail view
* Update API version
* Apply similar annotation to StockLocation API endpoints
* Extra tests for PartCategory API
* Unit test fixes
* Allow PartCategory and StockLocation lists to be sorted by 'pathstring'
* Further unit test fixes
