Inbox adjustments: Allow nested inboxes, add responsibleorg behavior and integrate separte inbox class.
Dropped automatic title for inboxes.

With the new implementation, it will exist a inbox for every orgunit, so the title
could be configured on every subinbox.
Inbox: Add redirecting to current subinbox (inbox of the current org unit).
Dropped no longer used `client_id` indexer for documents.

Because the inbox get nested now, and it will exist a subinbox for every org unit,
a mapping document <--> orgunit is no longer necessary, because it's given with the containment.
Reworked yearfolder functionality: observe subinboxes and the current inbox.

Additionally I've did some cleanup:
  - Wording
  - Move in to separte files nad into the inbox package
  - Add tests