Support reconfirming changed email addresses

[davewasmer]

The confirmable facet currently only supports confirming email addresses for initial registrations. But if users are able to change their email to an unconfirmed address, that mitigates many of the potential benefits of confirming emails.

The confirmable facet should hook into the save operation, and if the email is different, mark it as unconfirmed and send a confirmation email.

This depends on the core implementation / support of dirty state tracking (denali-js/core#228), or else we'll have to find a different approach to detecting the email change.