Forwarding Transitioncontroller: Fixed assign_to_dossier check.

Right now the `assign_to_dossier` (etc.) guard does not work correctly, when a forwarding has a responsible user without agency_group belonging. This results that the responsible user of the forwarding can't do any action.

This commit solves this problem.
Merge pull request #655 from 4teamwork/pg_inbox_guard_fix

Forwarding Transitioncontroller: Fixed assign_to_dossier check.
Dossier responsible field: use current admin unit's users, instead of only
current orgunit's users.

In a setup with multiple org_units per admin_unit, an limitiation that
only user assigned to the current org_units for the dossier responsibilty
has no additional benefit but rather lead to problems, when a user is
assigned to multiple org_units.