Allow anonymous users to unregister from entities

Functionality to allow anonymous registrants for a host entity to unsubscribe.

Registration provides users with the ability to contact registrants of a host entity via email (Broadcast).

Registrants associated with a host entity that are a Drupal account can simply delete their own Registration if they are given a 'delete own registration' permission. Anonymous registrants currently do not have a way to opt out of being contacted. Registration should allow anonymous registrants with a way to delete their existing registration.

Implementation

A submodule would be ideal as anonymous registrants may not be permitted in some setups.

• Add table with fields: Registration ID (primary/unique), random hash.
• When a new registration is created with anonymous person, a new row for above table is created.
• Add helper to generate unsubscribe URL for a Registration entity.
• Provide configuration setting: per- registration-type email signature with message along the lines of: 'Click here to unsubscribe from this event.
• Provide a token for inserting above unsubscribe signature. (for Rules etc.)
• Automatically add signature in Registration broadcast emails.

Alternatively: include the above unsubscribing functionality and move existing anonymous registrant functionlity from registration.module into the submodule.

A few form alters for registration_form, and implement a way to hook into registration_access_people().

I personally do not have any interest in this functionality, and may have not covered everything for this issue. Although I offer my time to implement it if required.