[META] GDPR Compliance

Summary of the comments so far is that the main features/changes needed are:

• 📌 Track history of subscribe/unsubscribe and proof of consent Fixed (#6) Includes an option to integrate with an existing module that supports consent tracking, specifically the GDPR module.
• #3037307: Improve experience for anonymous subscribers to modify existing subscriptions → (#7) revoking consent should be as easy as granting
• #3035370: Implement right to be deleted → (#7)
• #2996882: Tidy up unconfirmed subscriptions →
• ✨ Add "subscribed reason" explanatory text to newsletter Fixed
• ✨ Create GUI for subscription history Active