Convert webp images on upload

[joehoyle]

It's not unusual for people to upload webp images (especially easy when saving images from a browser that supports webp). Currently we don't even use Tachyon to serve webp, so no dynamic resizing, but also we have no handling for fallback format either. Given we support Tachyon/WebP already, it would be better by default to convert webp images to PNG on upload so get the benefits of dynamic resizing, and still be able to support legacy browsers. I'd suggest we set that to default behavior, and provide a module setting to disable it.