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.
We should be more explicit (e.g., provide an example and link to the RFC) about what a JKU should serve and what properties of a JWK should be set. I propose we try to be as explicit as possible and include `alg`, `kty`, `use`, and `kid`. `alg` will be restricted to `RS256` and `use` to `sign`. If the type or length of string in `kid` is restricted in any way, that should be documented too.