Add a reset for accurate next article calculation
Add a reset for accurate next article calculation

With the 80% threshold, the amount needed to scroll decreased with each new article load. This
caused new articles to load prior to the 80% threshold. The solution was to reset the `scrollTop`
_and_ the `calculatedAmount` values with the top offset of the new article (which is the sum of
height of all articles before the new one).

Also, I noticed that `$(document).height()` and `$("html").height()` produced different results; I
couldn't figure out why. The value for `$("html").height()` is accurate while the value for
`$(document).height()` is too much. This change improves the accuracy of the progress bar and the
loading of a new article.

Fixes lonelyplanet/destinations-next#793

lonelyplanet/rizzo-next,lonelyplanet/rizzo-next