Infer the redirect url from the request instead

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.

This switch statement doesn't do what I thought it does

Fail loudly if either the google_opts domain value or github_opts organization
values are not set in the configuration. The lack of these values means that
 a) in the Google case any @gmail.com address will be allowed
 b) the Github case any Github user will be allowed.

This was previously documented but left as a foot-gun in the code.

Future commits will allow for explicit wildcards to be set.