Implement full USWDS gov banner

[mejiaj]

The current gov banner is a simpler implementation. We should incorporate the full banner for correctness.

<div class="usa-banner">
  <header class="usa-banner-header">
    <div class="usa-grid usa-banner-inner"> <img src="{{ site.baseurl }}/assets/uswds/img/favicons/favicon-57.png" alt="U.S. flag">
      <p>An official website of the United States government</p>
    </div>
  </header>
</div>

source: https://github.com/GSA/plainlanguage.gov/blob/master/_includes/components/banner.html

[nick-mon1]

This will be closed out when site is migrated.