Overview: replace recently modified with ftw.activity stream

Removes the old recently modified from the overview tab and replaces it with an ftw.activity stream.

![bildschirmfoto 2014-09-03 um 14 08 06](https://cloud.githubusercontent.com/assets/7469/4133769/fbaf1bae-3362-11e4-8e60-ee94ff98c7fe.png)

@maethu