Use "static" templatetag insteadof the deprecated "admin_static" (#2288)

Author: luzfcb

templates/sponsors/admin/update_related_sponsorships.html

@@ -1,5 +1,5 @@
 {% extends 'admin/base_site.html' %}
-{% load i18n admin_static %}
+{% load i18n static %}
 
 {% block extrastyle %}{{ block.super }}<link rel="stylesheet" type="text/css" href="{% static "admin/css/forms.css" %}">{% endblock %}