This makes a couple of changes to Retina/high-DPI support, allowing for

crisper graphics on higher-DPI screens and preventing low-resolution

graphics from temporarily being used when higher-resolution graphics are

available on modern browsers.
The old retina.js script has been dropped in favor of the srcset

attribute. Modern browsers supporting srcset will make use of the best

graphics for the screen, even when moving windows between screens of

different DPIs, and unlike with retina.js, there won't be any need to

traverse the image elements and perform any graphics swaps. Older

browsers will use the new srcset shim in Djblets, which will traverse

the elements, but won't need to scan whether @2x images are needed by

performing HTTP HEAD requests.
Stylesheets now use the new .retina() in Djblets instead of its own

.at2x(), enabling support for SVG spritesheets for the icons on 3x and

higher screen DPIs.
It also switches the main logo to use srcset (preventing a jump from

low resolution to high) and fixes a syntax error with srcset for new

draft replies.

Tested various pages and made sure things looked right for 1x and 2x

image sets. Also temporarily turned off 2x, allowing SVG, and tested

that all icons scaled properly when zooming in.
Created a test page that tested each icon. Used it to verify all the

possible modes.
Tested that newly-created draft replies no longer caused errors in the

console and that they avatars appeared as high-DPI.
Loaded the page and saw that the Review Board logo didn't start with

the low-resolution image.
Tested that file attachment thumbnails were properly using the higher-DPI

thumbnails on a Retina display.