Allow scope override in provider_login_url template tag.

This propagates a "scope" argument in two places:

  1. OAuth2LoginView checks for ?scope= and adds it to the auth_params
     dict. Later on the auth_params dict will override the default scope
     in the url params dict in the redirect to FB.

  2. ProviderLoginURLNode handles 'scope' the same way as 'next', by
     propagating from the current GET query to the login URL. This allows
     /accounts/login?scope= to propagate properly, since that view is
     generic and wouldn't know what to pass in the template tag.

  1. ProviderLoginURLNode