Replace footer Github link with link to simplesamlphp.org

This is likely way more informative for the average person
clicking the link - and linking directly to github only
stimulates people asking help questions in issue tracker.

templates/_footer.twig

@@ -2,8 +2,7 @@
             <div class="wrap">
                 <div class="center copyrights dark-bg">
                     <br>Copyright © 2007-2018
-                    <br>
-                    <i class="fa fa-github" aria-hidden="true"></i> <a href="https://github.com/simplesamlphp/simplesamlphp">SimpleSAMLphp</a>
+                    <a href="https://www.simeplsamlphp.org/">SimpleSAMLphp</a>
                 </div>
                 <div class="logo-footer-right">
                     <div class="logo-footer">