migrations are not working with a custom user model

**auth.user** pollutes some schema migrations so the migrations with a custom user model are not working. 

**AUTH_USER_MODEL** and **AUTH_USER_MODEL_NAME** should be used everywhere or you could have some weird error like:

    Running migrations for basket:
     - Migrating forwards to 0006_auto__chg_field_basket_owner.
     > 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)

I've fixed the wishlist schema migration ([here the pull request](https://github.com/tangentlabs/django-oscar/pull/915))

order, basket and partner apps need the same fixes.

I'll try to create a pull request for this.