Convert all cardsets to PNG

[joeraz]

Since we've recently converted all the images in the main app to PNG, I'd like to do the same with the cardsets repo, and convert all of the GIF cardsets to PNG. PNG is generally a superior format, and is a standard these days, unlike when many of these cardsets were originally created. I think doing this will also help improve the image quality and possibly even improve the image loading performance.

Any objections to this plan?