If a site is configured for GitHub logins, _**and**_ has an email domain
whitelist, it's possible to get in a state where a new user is locked to
a non-whitelist email (their GitHub primary) even though they have an
alternate email that's on the whitelist. In all cases, the GitHub
primary email is attempted first so that previously existing behavior
will be the default.
- Add whitelist/blacklist support to GithubAuthenticator (via
EmailValidator)
- Add multiple email support GithubAuthenticator
- Add test specs for GithubAuthenticator
- Add authenticator-agnostic "none of your email addresses are allowed"
error message.
