NSFW clickthrough should have image preloaded

The image behind the NSFW clickthrough placeholder should be preloaded. I've had multiple occurrences where I clicked the NSFW banner on mobile and it didn't seem to do anything, so I clicked again and the result was that it registered a click on the image after it loaded and caused the image to open in a new window.

This could be an option if we are concerned that people will be unhappy with bandwidth being used to load images they don't want to look at anyway, but I'm not sure it's really that big of a deal; another user on the same instance may choose to look at that image and it could be seeded in the media proxy cache anyway.