Display commits and branches in which vulnerabilities where addressed

[fricklerhandwerk]

As a Nixpkgs user, I want to know on which branches a mitigation for a published vulnerability has landed and in which commit.

Depends on:

• Track relevant GitHub PRs #139
• https://github.com/NixOS/nix-security-tracker/issues/

Very ideally we'd even (at least enable to) produce a VEX statement (NixOS/nixpkgs#421125 (comment)).

Acceptance criteria

TODO

[fricklerhandwerk]

As a first step we could add a form field to published issues for adding those commits