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)

:cry: That would be a real shame, but can always be revisited.