add return link when create a group

templates/group/create_group.twig

 {% extends 'common/layout.twig' %}
 {% block content %}
+<p><a href="/groups">Return</a></p>
 <div class="form_format">
 <form class="ui form {% if formstatus %} {{formstatus}} {% endif %} " action="/groups" method="post">
 <h4 class="ui dividing header">Create a group</h4>