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_home.html

@@ -34,7 +34,11 @@
 		<li>
 			<h2>{% trans %}System health{% endtrans %}</h2>
 			{#- -#}
+			{%- if show_metrics -%}
 			<p>{% trans %}View the server status or send a broadcast message to all users.{% endtrans %}</p>
+			{%- else -%}
+			<p>{% trans %}Send a broadcast message to all users.{% endtrans %}</p>
+			{%- endif -%}
 			{#- -#}
 			<div>{% call standard_button("insights", url_for(".system"), class="primary") %}{% trans %}Manage system{% endtrans %}{% endcall %}</div>
 			{#- -#}