Make this plugin also an IAuthenticationPlugin which does all the job of
returning the user id.
It does not really make sense to delegate this to default authenticator.
A side effect is that token can still authenticate users with no
assignments, since tokens are scriptable, if this is a requirement, it
can be implemented in scripts.

Because this was broken, we took the liberty to introduce a breaking
change to fix naming, now type based scripts are getUserId.