SVG app icons don't load unless the URL ends in .svg

[deansheather]

Since there's no ImageSource that can load both .svg and bitmap images, and instead there's a separate SvgImageSource and BitmapImage, we currently only attempt to load SVGs when the URL ends in .svg. We don't have much control over the actual downloading/caching operations of these sources.

One potential solution is to do a HEAD request first to get the Content-Type of the image, but it won't be easy to cache that result.

Another would be to download/cache images ourselves.

Relates to #92