templates: login: Use /login/ as endpoint

Needed because otherwise the POST-ed data will be sent to a
redirect from /login to /login/, triggering an unnecessary
browser warning.

isso/templates/login.html

@@ -20,7 +20,7 @@ <h2>Administration</h2>
     <main>
       <div id="login">
         Administration secured by password:
-        <form method="POST" action="{{isso_host_script}}/login">
+        <form method="POST" action="{{isso_host_script}}/login/">
           <input type="password" name="password" autofocus />
         </form>
       </div>