[APPS-1282][PRI] add a setting sample app

:koala::+1:

This PR intends to add an app on the nav bar, demonstrating app settings.

<h2>REFERENCES</h2>
<ul>
  <li><a href="https://zendesk.atlassian.net/browse/APPS-1282">Jira</a></li>
</ul> 

/cc @zendesk/quokka