add ping groups to WPD sidebar

files/templates/sidebar_WPD.html

@@ -74,6 +74,8 @@
 		<a href="/h/music">Music</a>
 	</div>
 
+	<a class="btn btn-primary btn-block mt-5 mb-2" href="/ping_groups">PING GROUPS</a>
+
 	<div class="sidebar--rules sidebar--rules--wpd pt-2">
 		{% set rules = "rules_" ~ SITE_NAME ~ ".html" %}
 		{% include rules %}