Webpacker support

[gmq]

Any chance of releasing a package with the scss on npm to support activeadmin's new webpacker mode?

From a quick look the only issue would be the following line, since it can't find the image.

$skinLogo: #fff image-url("active_skin/default_logo.png") no-repeat 30px 0 !default

everything else should work as-is once published on npm.