Fix css-loader's resolving of /path/to/file type URLs

[jaredcwhite]

This is a 🐛 bug fix.

Summary

Previous versions (3.x) of Webpack'scss-loader wouldn't cause any problems if the file referenced in a stylesheet's url(…) was using a server-relative URL (aka /path/to/file.jpg). 4.x and beyond changed it so Webpack always tries to resolve the url within its own context. That doesn't work for us due to cases where /images/file.png should be loaded normally from the website's public folder (aka src), rather than pulled from somewhere in Webpack's frontend context.

This change makes it so if you use truly relative paths like ./something.jpg or even ../something/else.jpg, that will get resolved through Webpack (and in fact I've added file-loader support for images via Webpack now)…but any path starting with a slash now reverts to a passthrough so the browser can load that static file directly out of the site source.

TODO: Now that we actually do have image file support within Webpack, we could in theory locate images in frontend, rather than src, add import statements in either CSS or JS files, and then reference those with a Liquid tag/Ruby helper now that those files are in the Webpack manifest. It would then work similarly to Rails' Webpacker image tag support. A project for another day…

Fixes #239