[fix] Load CSS in extrastyle template block

Fixes a JS error caused by the override of
the extrahead block.

Author: nemesifier

openwisp_users/accounts/templates/account/password_change.html

@@ -4,13 +4,10 @@
 {% load static %}
 
 {% block title %}{% trans "Change Password" %}{% endblock %}
-{% block extrahead %}
-  <link rel="stylesheet" type="text/css" href="{% static 'openwisp-users/accounts/css/change-password.css' %}" />
-{% endblock %}
 
 {% block extrastyle %}
 {{ block.super }}
-{{ form.media.css }}
+<link rel="stylesheet" type="text/css" href="{% static 'openwisp-users/accounts/css/change-password.css' %}" />
 {% endblock extrastyle %}
 
 {% block content %}