Add config option for disabling upload of public third-party keys

By default, users can upload any OpenPGP keys to the configured keyserver.
With this option admins can disable the possibility to upload public
third-party keys to the keyserver so that users can only upload keys for
which they have the secret key.

(cherry picked from commit 711daad6fc94f4d5c5e2cde6f6d63920cd125573)

• GnuPG-bug-id: T7772