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/_WhoIsTypingTile.scss

@@ -61,7 +61,7 @@ limitations under the License.
 }
 
 .mx_WhoIsTypingTile_label > span {
-    background-image: url('../../img/typing-indicator-2x.gif');
+    background-image: url('$(res)/img/typing-indicator-2x.gif');
     background-size: 25px;
     background-position: left bottom;
     background-repeat: no-repeat;