Relation to user should use settings.AUTH_USER_MODEL

[arthur-wsw]

Relation to user models should use settings.AUTH_USER_MODEL instead of directly referencing the User model.

It generate two errors when making migrations:

ERRORS:
pinax_wiki.MediaFile.user: (fields.E301) Field defines a relation with the model 'auth.User', which has been swapped out.
HINT: Update the relation to point at 'settings.AUTH_USER_MODEL'.
pinax_wiki.Revision.created_by: (fields.E301) Field defines a relation with the model 'auth.User', which has been swapped out.
HINT: Update the relation to point at 'settings.AUTH_USER_MODEL'.