Change Slack to Zulip

Slack has been deprecated and replaced by Zulip. This represents those changes on our website by swaping out the links previously pointing to Slack now pointing to our Zulip.

Author: freeware-superman

_includes/footer.html

@@ -30,7 +30,7 @@
       <div class="col-4 col-lg-2 col-md p-3">
         <div class="row">
           <div class="col-12">
-            <a href="https://techaachen.slack.com" target="_blank" rel="noopener noreferrer" class="">Slack</a>
+            <a href="https://zulip.techaachen.de" target="_blank" rel="noopener noreferrer" class="">Zulip</a>
           </div>
           <div class="col-12">
             <a href="https://wiki.techaachen.de/" target="_blank" rel="noopener noreferrer" class="">Wiki</a>