Add icon and invite to new slack

This is just the raw invite link at this point. As soon as @bastelfreak
gets the shortlink updated, we'll replace it.

Tangent: we should simplify the nav header soon!

_includes/nav.html

@@ -20,6 +20,7 @@
     <ul class="navbar-nav ms-auto">
       <li class="nav-item"><a class="nav-link{% if page.url contains "/privacy-policy" %} active{% endif %}" href="{{site.url}}{{site.baseurl}}/privacy-policy"> Privacy Policy</a></li>
       <li class="nav-item"><a class="nav-link" href="https://github.com/voxpupuli"><i class="fa fa-github"></i> Github</a></li>
+      <li class="nav-item"><a class="nav-link" href="https://join.slack.com/t/voxpupuli/shared_invite/zt-2qps81elf-mp3bVZJZGqbZfKQTi4Q4bQ"><i class="fa fa-slack"></i> Slack</a></li>
     </ul>
   </div>
   </div>