minor: abstract user model

Warning: Compatibility break. Need a clear database.

Also in need of jbpasquier/slugs branches of djangoldp & djangoldp_profile

You have to add AUTH_USER_MODEL = 'djangoldp_account.LDPUser' to your settings.py

Note that on user save, user + account + chatprofile + profile slug will be updated with the one of the user.

@bleme If you can test it please.