Fix actor migration scripts
- Lookup for user_id instead of user_name, since user renames can cause old user names with no matching actor, as was the case for r_user_name field from user_relationship table at least. This may break for anonymous users (that will have user_id = 0). However, the functionality of this extension isn't mean to be used for anons, apparently.
- Do not instance a new user object. Instead, create one from the corresponding ID. Otherwise it creates users with a user name but user id equal to 0, causing getActorId to fail.
- Use DISTINCT to do the update once for each user id.