Fix ScrollList Error Computing Nearest Item

## Problem
When the gutter between pages in the ScrollList is large, calculating the nearest point on a page in the ScrollList when the event occurs between two pages can unfortunately return null rather than a point on the nearest of the two pages.

## Solution
While iterating through the list of pages in the ScrollList, keep track of which page is vertically closest to the event position in case the event occurs between two pages. Then compute the nearest location on that page.

## How to QA/+10
1. CI Build Passes
1. Check out the `fix_nearest_container_calculation` branch
1. Run `./init.sh && grunt qa`
The nested ternary is doing some of the same logic as the if - let's just use an if-elseif so it reads easier.