Added new type InboxContainer.

Used in setup's with multiple org_units per admin_unit, to store all inboxes (each for every org_unit).
Dropped Main- and Subinbox implementation, includings redirects on Maininbox etc.

This functionality is now covered by the InboxContainer type.
Added upgradestep which converts Main-Inboxes to InboxContainers.
HistoryViewlet Tests: Fixed isolation.

Reset `DownloadConfirmationHelper` state in `tearDown`, to avoid having inpacts of other tests.