never show a scrollbar

see the gray bars at the screenshot below? That's a tiny scrollbar because the avatar is to high. This PR fixes it. I think we should never have a scrollbar for single activities.

![screenshot](https://cloud.githubusercontent.com/assets/1589737/3628448/6d9b6aea-0e94-11e4-8fb9-20d58c828679.png)

cc @nickvergessen @jancborchardt @PVince81