Update BomItem 'validated' field (#4486)

* Adds new "validated" field to the BomItem model

- Previously this was dynamically calculated (very expensive)
- Now cached and updated whenever a BomItem instance is saved
- Will make the BOM API much more responsive
- Cleanup BomItem list API code also

* Adds data migration to update existing BomItem objects

* Exclude 'validated' field from BomItemResource class

InvenTree/part/admin.py

@@ -259,6 +259,7 @@ class BomItemResource(InvenTreeResource):
             'id',
             'part',
             'sub_part',
+            'validated',
         ]
 
     level = Field(attribute='level', column_name=_('BOM Level'), readonly=True)