Use double resolution logo for retina screens

Retina screens can display higher DPI images, and standard DPI images
(especially those containing text) can appear fuzzy.  This includes a
double size logo image, along with the CSS needed to display them.  To
make sure the 2x image matches the standard logo, a regenerated logo
image is included.  The responsive design for small screens has also
been adjusted to use the small logo instead of resizing the large logo.

This fixes #1221.