Add matrix link in footer of all the kde websites

Each website can also choose to link to a special room if they want

BUG: 428963

_includes/footer.html

@@ -58,6 +58,9 @@
 
     <section id="kSocial" class="container kSocialLinks">
         {% if site.social %}
+            % if site.social.matrix %}
+                <a class="shareMatrix" href="{{ site.social.matrix }}" rel="nofollow">Join us on Matrix</a>
+            {% endif %}
             {% if site.social.facebook %}
                 <a class="shareFacebook" href="{{ site.social.facebook }}" rel="nofollow">Post on Facebook</a>
             {% endif %}
@@ -89,6 +92,7 @@
                 <a class="shareInstagram" href="{{ site.social.instagram }}" rel="nofollow">Share on Instagram</a>
             {% endif %}
         {% else %}
+            <a class="shareMatrix" href="https://webchat.kde.org/#/room/#kde:kde.org" rel="nofollow">Join us on Matrix</a>
             <a class="shareFacebook" href="https://www.facebook.com/kde/" rel="nofollow">Post on Facebook</a>
             <a class="shareTwitter" href="https://twitter.com/kdecommunity" rel="nofollow">Share on Twitter</a>
             <a class="shareDiaspora" href="https://joindiaspora.com/people/9c3d1a454919ef06" rel="nofollow">Share on Diaspora</a>