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.
Can we and should we add some sort of "purpose" field to the token?  Something to indicate "this is a pre-verified account token, it is not valid for any other future use-cases where one might accept JWTs from marketplace for other purposes".