message: Add markup for app logo
diff: <form action="{{ action }}" method="post">
  +
  + {{#if logoUrl}}
  + <!-- show app logo -->
  + <fieldset><img src={{logoUrl}} style="max-width: 100%"/></fieldset>
  + {{#else}}
  +
  + <!-- show curveball logo -->
  + <fieldset style="background: #1e88e5;">
  + <img src="https://curveballjs.org/assets/images/logo.svg" style="max-width: 100%"/>
  + </fieldset>
  + {{/else}}
  + {{/if}}
  +
  {{# if msg}}
  <div class="msg info">{{ msg }}</div>
  {{/if}}
  