message: fix(side-nav): bring back hover effect
  This change fixes `<bx-side-nav>` that lost hover effect in its rail
  mode (`collapse-mode="rail"`).
diff: @@ -24,6 +24,10 @@ $css--plex: true !default;
  :host(#{$prefix}-side-nav[collapse-mode='rail']) {
  width: mini-units(6);
  +
  + &:hover {
  + width: mini-units(32);
  + }
  }
  :host(#{$prefix}-side-nav[expanded]),
  