Extend transition guards with agency-checks for the 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'
Merge pull request #626 from 4teamwork/pg_forwarding_agency_condition

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