Error when logging in with GitHub when email is not available

[asgrim]

When trying to log in with a GitHub account that does not have an email address, creating of the ThirdPartyAuthenticationData fails because the email is null.

Argument 3 passed to App\Service\Authentication\ThirdPartyAuthenticationData::new() must be of the type string, null given, called in /data/home/phphants/phph-site/src/App/Service/GitHub/GitHubAuthentication.php on line 73

[asgrim]

A couple of potential solutions I can think of:

• Reject the login with a message saying email is not available therefore that social login is not available
• Add an interim screen that lets the user enter their email address (as this is required for creating accounts) before completing the login/registration. This is probably the most ideal solution, and also handles the Meetup login which won't have emails (#146) if done in a generic way