Fix feature images from only displaying the first one loaded.

The image loader used by the feature image in app details is the same as the one used to load icons. The icon loader caches files on disk according to the filename in their URL (not their pathname). This is fine for icons, because each app has a differently named icon. However it is not fine for feature graphics, because they are all called featureImage.png. Therefore the first feature image is written to the cache with that name. When the next image is loaded, it checks the cache for featureImage.png, which does exist.

This change takes the entire path to the feature image on the server, removes anything except A-Za-z0-9._- and uses that for the cache name (for both icons and feature images).