Subscribe to a package affected by a vulnerability

[fricklerhandwerk]

As a Nixpkgs user, I want to be able to subscribe to updates on a package affected by a vulnerability, so I can track its state.

Depends on:

• Notifications overview #174

Acceptance criteria

Given a published vulnerability record,
When I subscribe to an affected package,
Then I will receive notifications on events related to that vulnerability record as affecting the subscribed package.

[florentc]

Implemented by #644