templates: Mark all void tags as self-closing.

This reverses the policy that was set, but incompletely enforced, by
commit 951514dd7d.  The self-closing tag
syntax is clearer, more consistent, simpler to parse, compatible with
XML, preferred by Prettier, and (most importantly now) required by
FormatJS.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

templates/500.html

@@ -2,7 +2,7 @@
 
 {% block customhead %}
 {{ super() }}
-<meta http-equiv="refresh" content="60;URL='/'">
+<meta http-equiv="refresh" content="60;URL='/'" />
 {% endblock %}
 
 {% block portico_class_name %}error{% endblock %}