Re-add `util` import to the sidebar

This defines `u-*` classes which are used in various places.

src/styles/sidebar/sidebar.scss

@@ -13,6 +13,7 @@ $base-line-height: 20px;
 // Base styles
 // -----------
 @import '../reset';
+@import '../util';
 @import './elements';
 
 // Misc. legacy :(