Support reduced userinfo data

The userinfo endpoint result for some authentification mechanisms is quite reduced. Especially the userinfo for github logins neither has an entitlement, nor a principle name.

Currently we fail with 500 response if we try to sync the entry with the IDL.

Instead of returning error responses we switch to add the username in the database. We still can't add them to groups, but at least it will not raise errors in the SMS login process anymore.

The fallback to the sub is also the very same strategy that the SMS backend has - so the sync with the subjects in the SMS backend db & the usernames in the IDL is still happening.