Opt-in to newsletter when using OmniAuth registration method

When a user registers through an OmniAuth method (external service), they are not automatically signed up for the newsletter. Even the mid-screen which asks for their name and nickname for the platform does not allow them to opt-in to the newsletter. This seems to be a problem for many admins since they don't have any sensible way to contact the platform users otherwise. Most of the people sign up using OmniAuth and it is not their first priority to go through their profile settings. Therefore, there should be the possibility to opt in for the newsletter during the OmniAuth registration. Preferably with a similar confirmation dialog as in the normal registration view.