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/zerver/config_error.html

@@ -9,7 +9,7 @@
             <div class="errorbox config-error">
                 <div class="errorcontent">
                     <h1 class="lead">Configuration error</h1>
-                    <br>
+                    <br />
                     {% if error_name == "ldap_error_realm_is_none" %}
                         {% trans %}
                         You are trying to log in using LDAP without creating an