keep slide sidebar content inside slide thumbnails

This patch stops slide content from displaying outside the slide's thumbnail.

Before:

![screenshot from 2014-12-16 13 10 20](https://cloud.githubusercontent.com/assets/364615/5458987/eb48df70-8524-11e4-91ba-27dfe670cea9.png)

After:

![screenshot from 2014-12-16 13 11 40](https://cloud.githubusercontent.com/assets/364615/5459014/1db6125c-8525-11e4-812f-92b72723854b.png)


(it also merges two .slideDrawer CSS rules into a single rule, for cleanliness)