Support custom user model with USERNAME_FIELD != "username"

[dpretty]

I'm using a custom auth model with USERNAME_FIELD = "email".

The changes here are enough, along with

USERENA_REGISTER_PROFILE = False
USERENA_REGISTER_USER = False

to get django-userena up and running with my project. However, I haven't done any thorough testing yet.

[coveralls]

Coverage increased (+0.02%) to 95.603% when pulling 416b1b8 on LaunchlabAU:master into 2ad1826 on bread-and-pepper:master.

[swistakm]

Thanks! This change looks very good but we won't be able to merge it until we resolve #535

[swistakm]

#535 that was blocking this PR from merging is finally resolved. Could you rebase your PR on current master so we can have it tested in Travis again?

When doing rebase you can also squash your changes in order to keep commit log more concise.

[sdether]

Should this change cover the scenario where the Custom User model does not even have a field called username? I rebased and tried this pull request and got the following error trying to create migrations with a User model with USERNAME_FIELD = 'email' and no username field at all.

SystemCheckError: System check identified some issues:

ERRORS:
<class 'userena.admin.UserenaAdmin'>: (admin.E033) The value of 'ordering[0]' refers to 'username', which is not an attribute of 'accounts.User'.
<class 'userena.admin.UserenaAdmin'>: (admin.E108) The value of 'list_display[0]' refers to 'username', which is not a callable, an attribute of 'UserenaAdmin', or an attribute or method on 'accounts.User'.