fix hitting back on nav, use more css over js for active styling (bug 1058996)

The behavior of the nav should remain the same. Except this fixes hitting "back" on desktop (i.e., there was duplicate pushStates, and hitting back didn't update the nav activness). Fixed by:

- remove duplicate navigation triggering
- defining the tab activeness in CSS using body[data-page-type] rather than CSS
Nope, just people saying "yeh we're gonna do it", but it never went to completion.

I agree, CSS should have been cleaner from the start. But it just started out as a namespace in a scratch CSS file, then it grew ugly, and then it had to become an extra task.