by hitting POST /<user_id>/email-code, we create an email two factor
code to send to the user. That email contains a link with a token that
will sign the user in when opened.
Also some other things:
"email verification" (aka when you first create an account) doesn't
hit the API anymore
refactor 2fa code verification and sending to use jsonschema, and share code between sms and email
Die marshmallow die!
