preVerifiedTokens

This is my initial sketch for #780

`preVerifiedToken` is a base64Url encoded JWS HS256 with the email address as the payload.

It uses a shared secret set in the config.
The issuer is another server, and expire times probably shouldn't be too short, so I think we can err on the side of simplicity here. If we see that we're having problems while testing the other end we can rethink it.