Bug 847168 - [User Story] Email Signature

This means that if anyone saved off account.identities[0], they're not holding a stale object with stale information.  Although identities are still fairly boring, we are pretty consistent everywhere else that the nested/child objects will be updated in-place.

The most-correct future-proof way to deal with this would be to build a map from identity.id to the identity instance.  Then use splice to empty out this.identities.  Then traverse wireRep.identities and find them in the map from their id.  __update them (add it to MailSenderIdentity, like we do here/elsewhere), then re-push them into this.identities.  In the super future we'd probably need/want events on them the identities themselves, but that's wayyy too future for now and probably needs some discussion with jrburke since we've thus far avoided dealing with nested collections, as it were.

Alternately, we can run with our existing hard-coding and just directly do this.identities[0].__update(wireRep.identities[0]).