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/_MemberList.scss

@@ -83,7 +83,7 @@ limitations under the License.
 
 .mx_MemberList_invite span {
     margin: 0 auto;
-    background-image: url('../../img/feather-icons/user-add.svg');
+    background-image: url('$(res)/img/feather-icons/user-add.svg');
     background-repeat: no-repeat;
     background-position: center left;
     padding-left: 25px;