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/stock/migrations/0081_auto_20220801_0044.py

@@ -0,0 +1,55 @@
+# Generated by Django 3.2.14 on 2022-08-01 00:44
+
+from django.db import migrations
+
+from InvenTree.helpers import constructPathString
+
+
+def update_pathstring(apps, schema_editor):
+    """Construct pathstring for all existing StockLocation objects"""
+
+    StockLocation = apps.get_model('stock', 'stocklocation')
+
+    n = StockLocation.objects.count()
+
+    if n > 0:
+
+        for loc in StockLocation.objects.all():
+
+            # Construct complete path for category
+            path = [loc.name]
+
+            parent = loc.parent
+
+            # Iterate up the tree
+            while parent is not None:
+                path = [parent.name] + path
+                parent = parent.parent
+
+            pathstring = constructPathString(path)
+
+            loc.pathstring = pathstring
+            loc.save()
+
+        print(f"\n--- Updated 'pathstring' for {n} StockLocation objects ---\n")
+
+
+def nupdate_pathstring(apps, schema_editor):
+    """Empty function for reverse migration compatibility"""
+
+    pass
+
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0080_stocklocation_pathstring'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            update_pathstring,
+            reverse_code=nupdate_pathstring
+        )
+    ]