Fix highlighting stories on the map when scrolling

[ebrelsford]

As a result of fixing #7, following the stories on the map now jumps jarringly from the story at the top of the screen to the story at the bottom of the screen.

[ebrelsford]

@almccon What do you think about this: once scrolling enters the final page (screen height), we calculate the percentage of the screen height scrolled and highlight the story proportional to that?

So as you're 1/3 of the way down the final page, we highlight the story 1/3 of the way down the current screen? By the time you're at the very bottom, the last story still gets highlighted?

I can't tell if it will be weird until I implement it, so curious to hear thoughts and alternatives!