Scroll position for historical pages

[bcardarella]

Visit page A, scroll down
From page A, click on Link to page B, scroll down
Back button to page A, previous scroll position is preserved.

Visit page A, scroll down
From page A, click on Link to page B, scroll down
From page B, click on Link to page A
Back button twice to page A, previous scroll position is not preserved.

[bcardarella]

To properly fix this I believe that you should not rely on window.location. Instead modifying Ember's history_location API. When the state is pushed: https://github.com/emberjs/ember.js/blob/c076329e429381925d2416a2fa29ee1e580c7ea7/packages/ember-routing/lib/location/history_location.js#L153

the state should also include a unique ID. This ID should be used to maintain the scroll positions.