Bug 833534: "X" in close button is inaccessible

This isn't ready to be deployed yet; I'd need to receive feedback from the defect reporter and assigned owner first. 

 - Added new class called `visuallyhidden`. This could be moved to a more global location for re-use
 - Added alternative text wrapped in span tag with class of `visuallyhidden` to the close link and to the three promo button links
 - Changed the 'x' close button to be an actual button, not just a span tag
 - Moved visual content to `after` tag. This may still get read out to screen readers, so there might be a better place for it. Maybe wrap it in a `span` tag with and `aria-hidden` attribute?

The design should match the previous look and feel; this was just a behind the scenes change.