redesign sidebar buttons

This makes two changes to buttons inside the sidebar

-  the buttons are stretched to the full width of the sidebar. This does only apply to buttons at the top level, so the e.g. "upload avatar" button centered on the avatar image is not affected.

-  The buttons above the avatar image in profile pages are moved below it.

The first change is especially important as it fixes currently misplaced buttons:

![message_button](https://f.cloud.github.com/assets/202576/1987271/51256252-8457-11e3-8d39-3f52573abba2.png)