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
this is an example of where we should use [OOCSS](http://www.smashingmagazine.com/2011/12/12/an-introduction-to-object-oriented-css-oocss/). any time you need to modify `.feed` or any of its children, you're going to get into specificity wars.

instead, we should add another class so `<ul class="feed">` becomes `<ul class="feed feed-splash">`. so we can then just target `.feed.feed-splash` (or even better `.feed-splash` depending on the selectors you've already defined).