OrgUnit Selector: store current unit to regular cookie instead of using the session_data_manager.
Reworked OrgUnit Selector:

- Differentiate between users_units (selectable) and admin_unit's org_unit (current org_unit).
- Change current org_unit functionality, the current orgunit has to be part of the current admin_unit.
Merge pull request #697 from 4teamwork/pg_rework_org_unit_selector

Rework org unit selector, for better support of multi admin_unit setups