Doorkeeper Improper Authentication vulnerability

OAuth RFC 8252 says https://www.rfc-editor.org/rfc/rfc8252#section-8.6

> the authorization server SHOULD NOT process authorization requests
> automatically without user consent or interaction, except when the
> identity of the client can be assured. This includes the case
> where the user has previously approved an authorization request
> for a given client id

But Doorkeeper automatically processes authorization requests without
user consent for public clients that have been previous approved.
Public clients are inherently vulnerable to impersonation, their
identity cannot be assured.

Issue https://github.com/doorkeeper-gem/doorkeeper/issues/1589

Fix https://github.com/doorkeeper-gem/doorkeeper/pull/1646