Replaced admin_media_prefix usage in mezzanine-templates

Hi Renyi,

It looks like the problem was just in the mezzanine_default templates.  I removed references to "adminmedia" and replaced the use of "{% admin_media_prefix %}" with "{{ STATIC_URL }}admin/" in the four templates where it was used.

There are three other templates that reference the ADMIN_MEDIA_PREFIX setting, but I left those alone.  They didn't interfere with my use of the templates.

Cheers,

Tom