Add icons to staff view

Fixes #372.

This adds icons for participants and comments to the staff's semester view.
Also the buttons were rearranged and the reviewed comments don't have a button anymore but are linked to the corresponding numbers.

![capture](https://cloud.githubusercontent.com/assets/1781719/3967597/b3661a38-27ae-11e4-8697-f602c71aa525.PNG)