Fix Donate button (loading in bursts)

[DeniseColbert]

Correct FOUC for sprites at page load.

@DeniseColbert Would you please provide a URL for reproducing this issue and a brief text description of the behavior noted?

[DeniseColbert]

www.freeukgenealogy.org.uk

This was the issue you spotted during our Meet, you might have said it was due to the way the donate button/image was constructed owtt/se? I can't capture it, but what happens is the button separates out into black shapes that are visible as the page is loading, but when it has finished loading the button is fine.

@DeniseColbert Ah right - this is a flash of unstyled content (FOUC) issue that we'll want to cover with CSS work.