Rather than having a UserPerson-specific confirmation, add an
EmailConfirmation object to allow multiple types of confirmations (eg,
opt-out requests in future).

To do this, we use a view (patchwork.views.confirm) that will call the
type-specific view with the confirmation object.

Also, add tests to check that the User/Person linkage system works.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>