Improve GitHub login for private sites

[erlend_sh (Erlend Sogge Heggen)]

@JaredReisinger

So… @erlend_sh (and team), what say you? Should I keep the email-whitelist-savvy GitHub logic isolated to a plugin, or is this something that makes sense in the builtin GitHub authenticator?

Sounds sensible but this is too low level for me to infer what impact this change would have. I’ll defer to the rest of the @team on this one.