Add !important to .hide

Currently, `_global.scss` is imported by `_alert-boxes.scss`, so the CSS output order is:

    .hide {
      display: none;
    }
    .alert-box {
      display: block;
    }

Which leads to unexpected behaviour, namely this div:

    <div class="alert-box hide">
        ...
    </div>

being currently visible!

Adding `!important` ensures that `.hide` will always override the default `display`.