Subinbox integration

Fixes #369 

Pyflakes cleanup.	ab0a51f
* Allow nested inboxes
* Add responsibleorg behavior to inboxes
* Integrate separate inbox class.
* Dropped automatic title for inboxes. …
* Inbox: Add redirecting to current subinbox (inbox of the current orgunit)
* Dropped no longer used `client_id` indexer for documents and adjust the different tabs and overview boxes
* Reworked yearfolder functionality: observe subinboxes, cleanup, wording, tests



See  referenced issue for more information.