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.
Merge pull request #656 from 4teamwork/pg_dossier_responsible_vocabulary

Dossier responsible field: use current admin unit's users, instead of only current orgunit's users.