Don't rescan_placeholders when loading fixtures

This should fix #1031.

This same fix may need to be applied to some of the other signal handlers too.

https://docs.djangoproject.com/en/dev/ref/signals/#pre-save

> `raw` -- A boolean; True if the model is saved exactly as presented (i.e. when loading a fixture). One should not query/modify other records in the database as the database might not be in a consistent state yet.