Add password-container and password-reset wrappers

This adds the wrappers necessary to center the content vertically and
horizontally within the page.

templates/zerver/reset_confirm.html

@@ -6,6 +6,8 @@
 
 {% block portico_content %}
 
+<div class="password-container">
+  <div class="password-reset">
     <div class="pitch">
         <hr/>
         <h3>{{ _('Reset your password') }}.</h3>
@@ -63,4 +65,7 @@
         <p>{{ _('Sorry, the link you provided is invalid or has already been used') }}.</p>
     {% endif %}
 
+  </div>
+</div>
+
 {% endblock %}