Update whitelisting

Whitelist Google users based on their email address instead of the username part of the email address.
Plain gmail (non Google Apps) accounts don't necessarily end in '@gmail.com', and whitelisting on username alone is open to abuse.
Skip testing for a Google Apps domain (ui.Hd) if no domain is configured.
Principals will still be added as the user part of the email address.

For the Github provider, skip checking that the user is a member of an organization is none is configured.