Add compatibility with custom user models

[ProfessorLogout]

When using a custom user model the username field may not exist on the model.

Django solves this by having a get_username method on the User model as documented here.

Since this is already required for a working custom user model I think it would be great to adhere to the same interface.

[lucasrcezimbra]

I forked the project to implement some features that I needed. Feel free to submit your PR there.

Source: https://github.com/lucasrcezimbra/ninja-api-key PyPI: https://pypi.org/project/ninja-api-key/