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/rooms/_TopUnreadMessagesBar.scss

@@ -54,7 +54,7 @@ limitations under the License.
     position: absolute;
     width: 38px;
     height: 38px;
-    mask: url('../../img/icon-jump-to-first-unread.svg');
+    mask: url('$(res)/img/icon-jump-to-first-unread.svg');
     mask-repeat: no-repeat;
     mask-position: 9px 13px;
     background: $roomtile-name-color;