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

(cherry picked from commit 1af81f8b3ea7ff95002ee51a0be5780f04ff080e)
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.

(cherry picked from commit 384dedcd4c7886223d23781b5eaed9ed04b528b5)
Merge pull request #698 from 4teamwork/pg_backport_697

Additional backports for bugfix release 4.0.6.