Tree query improvements (#3443)

* 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

InvenTree/part/test_part.py

@@ -220,6 +220,7 @@ class PartTest(TestCase):
 
     def test_category(self):
         """Test PartCategory path"""
+        self.c1.category.save()
         self.assertEqual(str(self.c1.category), 'Electronics/Capacitors - Capacitors')
 
         orphan = Part.objects.get(name='Orphan')