_includes/feedback.html

<li>
{% if site.feedback_text %}
  {% assign feedback_text = site.feedback_text %}
{% else %}
  {% assign feedback_text = "Feedback" %}
{% endif %}

{% if site.feedback_link %}
  <a class="email" title="Submit feedback" href="{{site.feedback_link}}">{{feedback_text}}</a>
{% else %}
  <a class="email" title="Submit feedback" href="#" onclick="javascript:window.location='mailto:{{site.feedback_email}}?subject={{site.feedback_subject_line}} feedback&body=I have some feedback about the {{page.title}} page: ' + window.location.href;"><i class="fa fa-envelope-o"></i> {{feedback_text}}</a>
{% endif %}
</li>