Fixed Django 1.5 requirements for admin media prefixes.

From Django 1.5 in advance, the template tags library ``adminmedia`` was removed. This leads to a ``TemplateSyntaxError`` exception when loading the admin, since some easymode templates makes use of this template tag for loading admin media.

This pull request adapts the easymode templates to the new Django 1.5 requirements, removing the ``adminmedia`` related template tags and replacing them by the new ``admin_static`` template tag library and the ``{{ STATIC_URL }}`` context variable.