REST login_url for external frontend

[rubengrill]

In #79 the LoginForm.save() accepts a parameter login_code_url with a default value of login_code_url, that will be used for reversing a url for the link in the email.

This means that a vendor either has to include both urls, or to provide a custom view just to have a matching url:

# include both urls, to handle the `login_code` server side, not via frontend
path('accounts/', include('nopassword.urls')),
path('api/accounts/', include('nopassword.rest.urls')),

# custom view, the url matches the url routing in the frontend application
path('accounts/login/code/', TemplateView.as_view(template_name='index.html'), name='login_code'),
path('api/accounts/', include('nopassword.rest.urls')),

It would be better if the vendor doesn't have to provide a custom view or to include both regular + rest views.
Maybe we should introduce some setting NOPASSWORD_LOGIN_CODE_URL, similar to django default setting LOGIN_URL, it could also be a url name or an actual url (anything that resolve supports).
This way, we would also support that the api and frontend are running on different machines, and the NOPASSWORD_LOGIN_CODE_URL would point to a url on a different server.