Restore vertical scrolling position

Hey Manuel,

I'm suggesting a small improvement which is to keep the vertical scrolling position when going back from comments to posts list or from a web view back to posts or comments list. Maybe it's just me, but I like to read HN sequentially and know where I was when I left a screen.

I also made a small optimization where the posts and comments lists are reloaded only if the font sizes have really changed.

Let me know if you have questions on this pull request.

Keep on the good work on your awesome app!