This provides Firefox (and possible other browsers) with a
high-resolution rich icon, in place of the previously removed
apple-touch-icon without having to use that one as it lacks an
alpha channel. This is the same method GH uses.
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/UI_considerations#Rich_icons
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: techknowlogick <matti@mdranta.net>