Add Slack logo with permalink to the channel

Author: fpotier

_includes/nav.html

@@ -83,5 +83,10 @@
     <li class="nav-item mx-lg-3">
      <a class="page-scroll nav-link" id="nav-link-documentation" href="{{'#documentation' | relative_url }}" onclick="onClick(event);">Documentation</a>
     </li>
+    <li class="nav-item mx-lg-3">
+      <a class="nav-link" href="https://join.slack.com/t/scicat/shared_invite/zt-251efp7t2-8IqFDo3sPN8TYWgKNywccg" target="_blank">
+        <img src="https://a.slack-edge.com/80588/marketing/img/icons/icon_slack_hash_colored.png" alt="Slack Logo" style="width: 24px; height: 24px;">
+      </a>
+    </li>
   </div>
 </nav>