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/_RoomSubList.scss

@@ -105,7 +105,7 @@ limitations under the License.
 .mx_RoomSubList_addRoom {
     background-color: $roomheader-addroom-color;
     color: $roomsublist-background;
-    background-image: url('../../img/icons-room-add.svg');
+    background-image: url('$(res)/img/icons-room-add.svg');
     background-repeat: no-repeat;
     background-position: center;
     border-radius: 10px; // 16/2 + 2 padding
@@ -120,7 +120,7 @@ limitations under the License.
 
 .mx_RoomSubList_chevron {
     pointer-events: none;
-    background-image: url('../../img/topleft-chevron.svg');
+    background-image: url('$(res)/img/topleft-chevron.svg');
     background-repeat: no-repeat;
     transition: transform 0.2s ease-in;
     width: 10px;