Extended NotificationForm to show members of groups on request.

* Added traversable method `get_users_for_group`
* Changed group table to use the group name as link text
* Javascript calls `get_users_for_group` on click, gets the corresponding members and displays them below the group name.

@maethu 

lower:
```
>>>'Mathias Leimgruber'.lower()
    mathias leimgruber
```

and a - z