Refactor Sidebar

The current state of sidebars is a bit chaotic as sidebar content may or may not be wrapped in a `sidebar_box` while styling is only applied to those.

In the first commit I changed the templates to consitently wrap contents in a `sidebar_box` except for some cases where I or Georg, our designer, thought they do not belong inside one (e.g. the follow button on proposal pages).

The second commit fixes the sizing of the `col3_content` element as this is the one the sidebar styling is now applied to. While the code in this commit is not beautiful I successfully tested it in IE9.

This PR makes it possible to style either `col3_content` to create a *sidebar* or `sidebar_box` to create several boxes as needed for the [publixphere theme](https://github.com/xi/adhocracy.publixphere_theme).