Don't check validation for images

At the moment, every time a badge is loaded, a revalidation is queued, which seems like overkill and also adds quite a bit of overhead when loading the recent validations page. I've tweaked this and things now seem to load much quicker.

However, I think this might be the wrong solution. I think it might be better to just display a plain image on the validations page, and still revalidate when the .png or .svg versions are shown. It's just overkill when loading multiple badges at the same time.