WIP: "Unsubscribe" link in notification emails is triggered by anti-virus software

Fixes #17198 (closed)

What does this MR do?

It makes the unsubscribe link to avoid people that is not logged in being unsubscribe directly on the first request. They will see a page with a unsubscribe link that will force the execution. The ?force=true param can be set manually on the unsubscribe link. The List-Unsubscribe header on the email is added so the email client can display the unsubscribe link on top. It will use the ?force=true param by default.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

What are the relevant issue numbers?

17198

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together