The current logo looks awful on high-density displays. SVG is a
better choice because it can scale to any resolution without
increasing file size.
Amending #2775 to add support for IE 8 by falling back to existing PNG
with img.onerror
Using relative URLs as directed by @btford and @petebacondarwin.
(commit by Brenton Simpson - @appsforartists)
Closes#2874
