As reported by @reger24, image and favicon viewing was broken with
unauthenticated requests on peers configured to require authentication
even from localhost.
So I unified viewing rights check in a single new function on
ImageViewer class.
Main image processing is now in ImageViewer, used by both ViewImage and
ViewFavicon.
Fixed URIMetadataNode.getFavicon to use non-standard icons with no size
ass fallback.