Fix unclosable active nav overlapping page content
Fix nav causing horizontal scroll bar to appear
Fix several site navigation issues

- Fix .viewed being delayed for focus events
- Use .viewed instead of :hover/:focus in skin css
- Remove unused colours from previous implementation in skin css
- Add a border at the bottom of each expanded nav to visually
  differentiate the nav from the page behind
- Fix 1 extra nav level being visible when a nav item with child navs
  was .active