Position the selected post more reliably below the header

Linked meta post: [The upper part of a post is hidden when the topic title is too long](http://meta.discourse.org/t/the-upper-part-of-a-post-is-hidden-when-the-topic-title-is-too-long)

It'd probably be better if we could just take the outer height of the header after the transformation performed when `showExtraInfo` is set on the header controller, but I don't think there's really a clean way to make that happen from `scrollTo()`.

I also considered triggering the initial scroll and then resampling the height and adjusting as necessary, but that ended up being far messier, and I was worried about it being jumpy.