Fix #1024. Add 'github' class to the 'Fork Me' banner.

doc/_templates/sidebarintro.html

@@ -5,5 +5,5 @@
   <li><a href="http://examples.flask-admin.org/" target="_blank">Flask-Admin Examples</a></li>
 </ul>
 
-<a href="http://github.com/flask-admin/flask-admin" target="_blank"><img style="position: fixed; top: 0; right: 0; border: 0;"
+<a class="github" href="http://github.com/flask-admin/flask-admin" target="_blank"><img style="position: fixed; top: 0; right: 0; border: 0;"
    src="//s3.amazonaws.com/github/ribbons/forkme_right_darkblue_121621.png" alt="Fork me on GitHub" /></a>