Add support for overriding forms

[monosans]

Such a setting exists in allauth. I miss it very much here.

[valberg]

Hey @monosans!

Good idea! Can you add some tests to make sure that we are not breaking anything (now and in the future).

[akx]

I'm a tiny bit on the fence about this – on one hand, in the spirit of avoiding There's More Than One Way To Do It, I'd just say you should derive the views in your app with a custom form_class, but since allauth already does this, it makes sense to follow the same thing here 🙂