Allow to switch jwt access token verification and off. Many (most?) IDP
provide JWT encoded access tokens. If ocis is configure to assume jwt
access tokens (access_token_verify_method==jwt) we now properly verify
the tokens signature and a set of standard claims ("exp", "iat" and nbf"
by way of the jwt module's standard verification and "iss" explicitliy).
This change also allows for introduction of other access token verification
mechanism in the future (e.g. through introspection (RFC7662).
Up to now the builtin lico was using the "username" as the login
attribute, while the proxy (and to some extend the auth-basic) service
tried to uniquely identify users by mail address. This aligns the
default configuration of the services to use the username everywhere.
Fixes: #4039
