[activities] Cannot restore activities

As of today, the restore of actions from the app actstream is broken. The reason is our backup/restore functionality depends on apps fixture dumping using natural keys instead of database identifiers. We cannot guarantee that the restored objects will have exactly the same PKs all the time.

The app actstream does not use natural keys for fields such as actor_object_id, and target_object_id so, when restoring, it relies on the exact usage of a given PK and it gets lost. This may cause degradation on the admin page:

Or more severe errors on the front page:

@samuel.gaist: Suggestions on how to proceed?