Changes

* It is likely that a user will have the same person represented across different contact providers. We should allow the user to merge these duplications into an integrated view, either manually, or via an automated process. Edits to a contact should then be propagated to the contact providers that compose that contact.

|Feature users and use cases=This change will affect all Thunderbird users who make use of the address book.

|Feature non-goals=At this point, we're not going to let contact providers push changes to other contact providers through Thunderbird. So, in other words, if a contact provider X sends an updated field, Thunderbird will not update this field for other contact providers.