* Current OIDC implementation asserts that user email check must come
from JWT token claims. OIDC specification also allows for source
of user email to be fetched from userinfo profile endpoint.
http://openid.net/specs/openid-connect-core-1_0.html#UserInfo
* First, attempt to retrieve email from JWT token claims. Then fall back to
requesting email from userinfo endpoint.
* Don't fallback to subject for email
https://github.com/bitly/oauth2_proxy/pull/481