Assign to dossier fix

This PR contains some fixes for the AssignToDossier functionality:
* use Yearfolderstorer instead of manually moving th e forwarding to the yearfolder.
* Admin_unit intern `remote_requests` uses now unrestricted traversing. It's not be secured that all parts from the traversing path, are available for the user. But because the remote request is only used from other
intern functions and the security is checked otherwise, it makes sense to use an `unrestrictedTraverse`.

@lukasgraf please have a look ...