Convert 'part" to MPTT model - based on the 'variant_of' field

- Now recursive variants can be implemented properly

InvenTree/part/migrations/0039_auto_20200515_1127.py

@@ -0,0 +1,50 @@
+# Generated by Django 3.0.5 on 2020-05-15 11:27
+
+from django.db import migrations, models
+
+from part.models import Part
+
+
+def update_tree(apps, schema_editor):
+    # Update the MPTT for Part model
+    Part.objects.rebuild()
+
+
+def nupdate_tree(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0038_auto_20200513_0016'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='level',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='lft',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='rght',
+            field=models.PositiveIntegerField(default=0, editable=False),
+            preserve_default=False,
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='tree_id',
+            field=models.PositiveIntegerField(db_index=True, default=0, editable=False),
+            preserve_default=False,
+        ),
+
+        migrations.RunPython(update_tree, reverse_code=nupdate_tree)
+    ]