css: Revert sidebar-menu-icon specificity change.

3ac0c3c401 incorrecly removed
specificity declarations needed to prevent that CSS from applying to
the right sidebar, which hasn't been properly prepared for this
change.

web/styles/left_sidebar.css

@@ -896,7 +896,8 @@ li.top_left_scheduled_messages {
     All of our left sidebar handlers use absolute
     positioning.  We should fix that.
 */
-.sidebar-menu-icon {
+.bottom_left_row .sidebar-menu-icon,
+.top_left_row .sidebar-menu-icon {
     display: none;
     width: 100%;
     cursor: pointer;