Fix the position of the banners

Fixes bugs with the first position in parallax banners.

https://trello.com/c/Ctzkwi65/326-parralax-headers-and-weekly-photos-often-broken-in-safari
https://trello.com/c/TfAmgTAx/341-banner-in-interests-not-displaying-correctly-again


this is because of safari, when you refresh the page and the page is scrolled down to the position that you were before the refresh. I'm going to explain whats happening:

Steps:
1.- Load an interest page
2.- Scroll down so you can view one of the banners
3.- At this position, refresh the page
4.- Here is where the problem appears: When the page is refreshed, this is what happens:
--> The page is loaded (no javascript yet). There is no scroll, the page loads at the top.
--> Then, Javascript loads.
--> Then, after the javascript is executed, the browser scrolls the page to the position that you were before the refresh. 

Because of that, the initial position of the banner images with parallax is wrong, because the initial position script is executed when the page is at the top (before the browser updates the scroll). 

Waiting for the load event it fixes this issue, because is fired after the browser scrolls the page, so the position is calculated when the scroll is at the right position.