Extend transition guards with agency-checks for some forwarding transitions:

The following transitions should also be available, as agency transition,
when the user is a agency member of the assigned org_unit.

* `forwarding-transition-assign-to-dossier`
* `forwarding-transition-reassign`
* `forwarding-transition-close`
* `forwarding-transition-reassign-refused`

@lukasgraf please have a look...