Scroll post-item-view into view

Closes #3998
- Scroll the active post into view on page load
- If the active post changes and the newly active post is offscreen, scroll it into the middle of the screen

This means keyboards trigger infinite scrolling now!

![keyboard scrolling](https://cloud.githubusercontent.com/assets/1207005/5022999/41261acc-6aac-11e4-9bdd-f541ca8b79c9.gif)

@PaulAdamDavis I could just give up entirely on scrolling the item into view on page load by removing this line. That way, it just cares about moving it into view as you keyboard your way through...