Replaced `ClientsVocabulary` with the `OrgUnitsVocabulary`.

According to this changed the `ResponsibleClientVocabulary` is renamed as well.
Define current_org_unit as default for task `responsible_client`.
Replaced `ClientsVocabulary` with the `OrgUnitsVocabulary`.
Task: Reworked single-orgunit behavior in task forms.

The `responsible_client` field should only be available if there
are mor than one orgUnits. The earlier implementation has checked
the number of clients.