add support for builtin login to the authorization form

[edemocracy]

add support for builtin login to the authorization form
and add AuthorizationLoginView which enables it

[edemocracy]

I'd prefer to make this the default behavior, and to remove LoginRequiredMixin from BaseAuthorizationView, but this would be backwards incompatible. What do you think?

[coveralls]

Coverage remained the same when pulling 89f1834 on edemocracy:feature/authlogin into 0b3712e on evonove:master.

[outsbart]

Thanks for your pull request. Altough this is an interesting way of solving the problem, I believe we should adopt a more configurable and already proven-to-be-good approach instead of using a non-standard one.