Replace {% admin_media_prefix %} tag with {{ STATIC_URL }}admin.

 - Django 1.5 deprecates the adminmedia tags library.
 - See bullet no. 8:
 https://docs.djangoproject.com/en/1.5/releases/1.5/#miscellaneous
PEP8 Update models.py
PEP8 Update admin.py
PEP8 Update cms_plugins.py
PEP8 Update rest of files.