Don't use a special redirect to generate thumbnails for SVG images

[bhousel]

Since we started tracking flags in #4745 ...
They don't look very good on nsi.guide because we are using specialredirect to generate thumbnails see here

The flags are all available in SVG, so the browser should really just fetch that directly.
We should check whether a commons image is an SVG, and generate the correct url to just fetch it.