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


Makes sense. Is it definitely the onload event that the browser waits for before scrolling? Or could it be earlier, like domContentLoaded?

The only reason I ask is that onload will be delayed until we get the ads back, and if that resource takes a while then it will delay your script executing.