core: fix error 500 with expired reset password links

core/templates/core/password_reset_confirm.jinja

@@ -1,10 +1,14 @@
 {% extends "core/base.jinja" %}
 
 {% block content %}
+{% if form %}
 <form method="post" action="">
 {% csrf_token %}
 {{ form.as_p() }}
 <input type="submit" value="{% trans %}Reset{% endtrans %}" />
 </form>
+{% else %}
+{% trans %}It seems that this link has expired. To generate a new link, you can follow this link: {% endtrans %}<a href="{{ url('core:password_change') }}">{% trans %}lost password{% endtrans %}</a>.
+{% endif %}
 {% endblock %}