UI: show content tab as selected when landing on petition edit page

The tab was shown as selected only when clicked, but since this is
the default tab it should be shown as selected by default.

pytition/petition/templates/petition/edit_petition.html

@@ -51,8 +51,8 @@
 </div>
 {% endif %}
 <div class="list-group list-group-horizontal mb-5" id="list-tab" role="tablist">
-    <a href="#content_form" class="list-group-item list-group-item-info list-group-item-action"
-    data-toggle="list" aria-controls="content_form" role="tab"><span class="oi oi-clipboard"></span> {% trans "Content" %}</a>
+    <a href="#content_form" class="list-group-item list-group-item-info list-group-item-action active"
+    data-toggle="list" aria-controls="content_form" role="tab"><span class="oi oi-clipboard" aria-current="true"></span> {% trans "Content" %}</a>
     <a href="#style_form" class="list-group-item list-group-item-info list-group-item-action"
     data-toggle="list" aria-controls="style_form" role="tab"><span class="oi oi-brush"></span> {% trans "Style & Look" %}</a>
     <a href="#social_network_form" class="list-group-item list-group-item-info list-group-item-action"