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/structures/_MyGroups.scss

@@ -54,7 +54,7 @@ limitations under the License.
 
     &:before {
         background-color: $accent-fg-color;
-        mask: url('../../img/icons-create-room.svg');
+        mask: url('$(res)/img/icons-create-room.svg');
         mask-repeat: no-repeat;
         mask-position: center;
         mask-size: 80%;