Changed Slack link to Mattermost link (#289)

* Changed Slack link to Mattermost link

I have not been able to run it on my system, but I didn't really change anything other than text.

* Use HTTPS by default

content/about/contact.erb

@@ -14,9 +14,9 @@ contact_links:
     link: https://twitter.com/zeuswpi
     description: Twitter
     action: Follow us
-  - icon: slack
-    link: https://zeuswpi.slack.com/
-    description: Slack
+  - icon: comments
+    link: https://mattermost.zeus.gent/
+    description: Mattermost
     action: Message us
   - icon: inbox
     link: http://lists.zeus.ugent.be/mailman/listinfo/leden
@@ -108,14 +108,13 @@ contact_links:
     </div>
   </div>
 
-  <!-- Slack information -->
-  <div id='slack-info' class='tile is-parent'>
+  <!-- Mattermost information -->
+  <div id='mattermost-info' class='tile is-parent'>
     <div class='tile is-child box content'>
-      <h1 class='title has-text-centered'>Slack info</h1>
+      <h1 class='title has-text-centered'>Mattermost info</h1>
       <div class='is-divider'></div>
-      <p id='slack-info-text' class='is-size-5'>
-        You can register with any <strong>*@zeus.ugent.be</strong> or even <strong>*@ugent.be</strong> email address.
-        If you don't have one, try to reach us on one of our other channels and ask for an invitation.
+      <p id='mattermost-info-text' class='is-size-5'>
+        You can register with any valid email address.
       </p>
     </div>
   </div>