Tweak primary navigation

Created by: tysongach

• Rename sidebar to navigation, which is based on its purpose rather than its visual placement.
• Use nav element.
• Remove unnecessary ul; the nav provides enough semantics to wrap the links in.
• Tweak visual design: show a light background on hover, add more whitespace between navigation links and main content.

After this PR:

(“Orders” is being hovered)

Before: