This introduces a series of improvements and fixes for high-DPI (Retina)

support across Djblets, focusing primarily on support for srcset

attributes and the usage of @3x (or higher) and SVG image files in

CSS.
The Avatar srcset support has been made more general. Instead of

focusing entirely on avatars, the new support works on all image

elements. There's a few key differences in the new method:


The srcset capability check has been fixed. The old check attempted

   to call get() on the test element, which is actually intended to

   perform an HTTP request. The result was that srcset was never

   marked as supported. The intent was to look up the DOM element from

   the list, which has been fixed.


The new method no longer attempts to parse an empty srcset or

   change the image URL if the pixel ratio hasn't changed, reducing the

   workload for images without srcset or when repeatedly called.


The new method is also a standalone method, rather than a jQuery

   method, lazily performing the DOM query only if needed, improving

   performance on modern browsers that natively handle srcset.


The old avatar method, introduced within the 0.10 release cycle, has

been removed. The only effective difference it had with the new support

was the addition of a CSS class, which wasn't used anywhere.
The old .at2x CSS mixin has been replaced with a new .retina mixin.

This one is based on the latest mixin from retinajs.com, but heavily

modified to allow usage of SVG files. When enabled, screen pixel ratios

higher than the highest ratio level (2x, 3x, etc.) specified will use

the SVG file instead, ensuring quality on all screen resolutions. The

mixin for this has moved to a new file (css/mixins/retina.less), and a

a compatibility file and compatibility macro have been added.
The integrations list has also been updated to use srcset.

Tested that the srcset support capability was reporting the correct

value.
Tested that .at2x() continued to work as it did before.
Tested the new .retina() with multiple density levels and with SVG

support.
Tested that integrations were setting srcset correctly.
Tested that datagrids were using SVGs when limiting the max density

level to 1 (forcing SVGs for Retina displays).