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/css/views/elements/_ImageView.scss

@@ -50,7 +50,7 @@ limitations under the License.
     max-height: 100%;
     /* object-fit hack needed for Chrome due to Chrome not re-laying-out until you refresh */
     object-fit: contain;
-    /* background-image: url('../../img/trans.png'); */
+    /* background-image: url('$(res)/img/trans.png'); */
     pointer-events: all;
 }