migrations are not working with a custom user model

There are still references to **auth.user** in some schema-migrations so migrations are not working with a custom user model.

**AUTH_USER_MODEL** and **AUTH_USER_MODEL_NAME** should be used everywhere or you could have some weird error like:

     > order:0001_initial
     > offer:0001_initial
     > voucher:0001_initial
     > basket:0001_initial
     > basket:0002_auto__add_field_line_price_incl_tax
     > basket:0003_auto__add_field_line_price_excl_tax
     > basket:0004_auto__add_field_line_stockrecord
     > basket:0005_auto__add_field_line_price_currency
    DatabaseError: value too long for type character varying(50)

order, basket, partner and wishlist apps should be fixed.

I'll try to create a pull request for this.