As administrator I want to restrict notification messages sent from my self-hosted mattermost server to be GPG signed and encrypted.

That means, if a user hasn't uploaded a GPG public key to his account, notifications sent to him only contain some metadata or nothing at all.
If a user has uploaded a GPG public key, then the notification message is encrypted for him as receiver, so only he can read it.
The notification message shall be signed in any case with the private key that I configured in my mattermost server.

Motivation: transport layer security between the mattermost server and clients is a great thing for transfering confidential information, however, if the receiver happens to be offline/away and notifications are configured to contain the content of the message it results in a leak of this sensitive information because the data lands in plain text on foreign mail servers.