[web, formatting] move Interval data type display logic from final view to printable_value (closes #1984742) also, fix implementation so that negative time delta are properly considered

<div id="ajaxroot">
  <div class="ajaxHtmlHead">
    <script src="http://foo.js" type="text/javascript"> </script>
  </div>
  <h1>Hello</h1>
</div>