Sidebar markup tweaks

Just some cleanup.

The `ul` in the sidebar didn't make semantic sense, and wasn't really contributing to styling. Replaced with a `div#bio` (open for discussion). Also removed the `hr`s (though I could keep a style rule for `.sidebar hr` for flexibility/backwards compatibility), adding borders on `div#bio` instead. And while I was at it, removed the wrapper `div` around `div#social`, which doesn't seem to serve any purpose.

Rendering shifts of a few px related to the margins on that `ul`.