Define SCSS variable for resource dir

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

res/themes/light/css/_paths.scss

@@ -0,0 +1,3 @@
+// Path from root SCSS file (such as `light.scss`) to `res` dir in the source tree
+// This value is overridden by external themes in `riot-web`.
+$res: ../../..;