Inbox and forwarding rework

This PR includes all necessary commits for the two issues #90 and #91, which are the following:

### Inbox views/tabs rework:
* Dropped tabs `assigned_forwardings`, `given_tasks` and `assigned_tasks`
* Implements new tabs `assigned_inbox_tasks` and `issued_inbox_tasks`
![bildschirmfoto 2013-09-17 um 07 56 33](https://f.cloud.github.com/assets/485755/1154968/0603c362-1f5e-11e3-86ae-4ac9a128a7c7.png)
* Implements new box for the inbox overview:
![bildschirmfoto 2013-09-17 um 08 02 13](https://f.cloud.github.com/assets/485755/1154994/d072907e-1f5e-11e3-9f67-8b6d03ad711f.png)
* Added new tab `closed_forwardings` used in yearfolders.

### Additional and reworked forwardings functionality:
* Add a refuse forwarding form including an optional response field.
![bildschirmfoto 2013-09-17 um 08 11 26](https://f.cloud.github.com/assets/485755/1155026/09aec0aa-1f60-11e3-8d50-0ce42a64dba3.png)

* When refusing a multiclient forwarding:
 * It stores an closed copy to the actual yearfolder of the current inbox.
 * Resets the responsible of the forwarding to the issuing inbox group.
 * Set in to the new state `forwarding-state-refused`.
 * ![bildschirmfoto 2013-09-17 um 08 21 55](https://f.cloud.github.com/assets/485755/1155067/7fd4ad3e-1f61-11e3-9bfa-6d50313858d4.png)
* Add a new reassing-refused-forwarding wich allows you to select another responsible client and responsible inbox.
![bildschirmfoto 2013-09-17 um 08 23 32](https://f.cloud.github.com/assets/485755/1155075/b2552770-1f61-11e3-936c-9759f8a3c3f3.png)