Fix hardcoded iframe on same domain
Switch to pluggable User models.

- Update all foreign keys to use settings.AUTH_USER_MODEL
- Fix migrations accordingly
- Replace a direct inheritance with an indirection through a OneToOne
  field to settings.AUTH_USER_MODEL
[Django1.6] Fix calls to (removed) django.conf.urls.defaults.
[Django1.6] Fix parent of CMSPluginBaseMetaclass.