admin: allow disabling display of metrics

This is useful in situations where the admins of the Snikket
server (i.e. those who care for the docker containers) are not the
same people as the people who are admins of the Snikket service
(i.e. those who care for the users).

snikket_web/templates/admin_system.html

@@ -2,6 +2,7 @@
 {% from "library.j2" import form_button %}
 {% block content %}
 <h1>{% trans %}Manage system{% endtrans %}</h1>
+{% if show_metrics %}
 <h2>{% trans %}Overall system status{% endtrans %}</h2>
 <div class="elevated el-2">
 	<dl>
@@ -77,6 +78,7 @@
 		</dd>
 	</dl>
 </div>
+{% endif %}
 <h2>{% trans %}Broadcast message{% endtrans %}</h2>
 <form method="POST">{{ form.csrf_token }}<div class="form layout-expanded">
 	<p class="form-desc">{% trans %}This form allows you to send a message to all users currently online on your Snikket server. Use it wisely.{% endtrans %}</p>