Add configuration mixins for allauth

[jbeezley]

Putting this here for discussion. I'm not sure where to divide settings that go here vs. in the cookiecutter example. What is here provides authentication related views (sign up, login, change password, verify email, etc.) that works fairly seemlessly with Google OAuth. A few things have to be manually configured that can be documented in the cookiecutter.

1. Configure the site at http://localhost:8000/admin/sites/site/
2. Configure the Google oauth at http://localhost:8000/admin/socialaccount/socialapp/

From there, the server rendered views (e.g. http://localhost:8000/accounts/login/) work as expected.

What doesn't work is the redirection after login, which defaults to the non-existent /accounts/profile page. I'm not sure how to handle this in general as the SPA client will want to have control over it. We could store the desired redirect target in a cookie or the oauth state, as we've done elsewhere. In any case, that code probably belongs here.

To what degree do we want to make decisions about defaults here? Should we even add the google provider at all? Should it be a different mixin?

Another question is about styling the templates... should this library attempt to make the pages look okay, or is that purely a concern for the consumer?

[jbeezley]

I don't have permission to merge.

Still changing.