Adds a `$res` SCSS variable set to the path from the root SCSS file to the `res`
directory.
This is a different base path than previously used in CSS URLs (it goes up 3
directories instead of 2), because Webpack will now be resolving images relative
to the root SCSS file, so the path corresponds to a source tree location,
instead of a path in the build output tree.
Defining this variable has two main goals:
* URLs are a bit easier to read
* The path can be overridden, which is needed for external (riot-web) themes
This means that themes which include `light/css/_base.scss` (currently Dark and
Status) won't be forced to have Light's font-faces included. This only really
matters for Status, which uses different fonts throughout.
