Allow configured users to be authenticated locally

+ Create `isExplicitLocalUser` method, which checks configuration
  and returns true if the given username is listed under the
  `local_users` key.
+ Use `isExplicitLocalUser` to provide overrides to LDAP and ExtDB
  authentication methods.
+ Give a variable type to `$opo_auth_config` for IDE hinting.
Merge pull request #96 from leftclickben/GR355-9797/AllowExplicitLocalUsers

Allow configured users to be authenticated locally