Handle "showmigrations" command (#6062)

- If provided, do not load custom app ready code
- Ref: https://github.com/inventree/InvenTree/issues/5804#issuecomment-1785562383

InvenTree/InvenTree/ready.py

@@ -49,6 +49,7 @@ def canAppAccessDatabase(allow_test: bool = False, allow_plugins: bool = False,
         'prerender',
         'rebuild_models',
         'rebuild_thumbnails',
+        'showmigrations',
         'makemessages',
         'compilemessages',
         'backup',