Move sidebar into empty space

[kgilles]

With wider monitors there's a lot of empty space to the right of the post feeds. We want to utilize this space when possible by rendering the sidebar next to the post feed container rather than inside of it.

We're also looking to clean up the sidebar design a bit. As can be seen in the screenshot there's some padding that should be corrected for the admin badges, and borders that don't always go full-width. Some general polish would be nice.

Also in the mobile view the sidebar currently renders above the post feed when opened. @kgilles believes that it might look and feel better if we render it on top of the post feed, or give it its own page like Reddit does. But this is up for discussion.