Support allowed_users and admin_users

[consideRatio]

allowed_user and admin_users are config options from the Authenticator base class. They should work in this authenticator class as well, allowing usernames to be listed in order to be allowed access.

[consideRatio]

I think #269 is a step towards making sure that allowed_user and admin_users works more as expected, where for example users doesn't have to be listed both in a allowed group and individually in allowed_users.

[minrk]

#269 actually doesn't fix allowed_users (yet) but it's a tiny change to do so. I'll make sure there's a test.

[minrk]

This is now fixed and tested by #269