Fix templatetag compatibility with Django 1.5

According to [1] Django 1.3 introduced the staticfiles application
and as of Django 1.5 adminmedia.py is no longer prestent in 
django.contrib.admin.templatetags. The use of {% STATIC %} from 
django.contrib.admin.templatetags.admin_static is required.  

[1] https://docs.djangoproject.com/en/dev/releases/1.4/#django-contrib-admin