Fix templatetag compatibility with Django 1.5

Django 1.3 introduced the *staticfiles application* and since Django 1.5 `adminmedia.py` is no longer present in `django.contrib.admin.templatetags`. The use of `{% STATIC %}` from
`django.contrib.admin.templatetags.admin_static` is required.

See also https://docs.djangoproject.com/en/dev/releases/1.4/#django-contrib-admin 
and https://docs.djangoproject.com/en/dev/releases/1.5/#miscellaneous