remove call to  from WithNextUrlMixin.get_success_url

I was attempting to comply with code found in
`django.contrib.auth.views.login`
(https://github.com/django/django/blob/master/django/contrib/auth/views.py#L42).
In revisiting that code, I clearly misread what was going on there and
the call to `resolve_url` is not necessary.
PasswordResetConfirm should raise an AssertionError when  or  is missing

`django.contrib.auth.views.password_reset_confirm` does this, so we
should too.