Support emailless signup with OAuth

Problem to solve

Now if OAuth provider doesn't grant access to a email, signup fails.

For example in the case of GitHub I get the following message:

Could not authenticate you from GitHub because ": {"message":"not found","documentation url":"https://developer.github.com/v3/users/emails/#list-email-addresses-for-a-user"}".

I surely don't want to disclose my email address to everyone who have set up a GitLab instance and hosts own projects there to which I need to contribute.