Refactor sidebar CSS

This refactors the CSS of the sidebar to be simpler and easier to manage.

Many of the rules applied to the sidebar were from the 'nav' class making things look more like buttons, then the 'nav-list' class undoing most of that.  An easier solution is to just remove the nav class, and many of the other rules can be removed.  It also cleans up the markup a bit, and adds a number of icons to the sidebar.