contacts: Remove ContactEntry and add Unsubscription

This change removes the unused `ContactEntry` model and the `subscribe` view and
replaces it with an `Unsubscription` model and an `unsubscribe` view. It works
similarly, but is intended to be used with the `list-unsubscribe` and
`list-unsubscribe-post` headers.

Use decorator syntax to register admin handlers

Same-same, just marginally neater.