Extract some email address utility code from the receiver stack

Summary:
Ref T7477. We have some address normalization code in the reciever stack that is really shared code. I want to introduce some new callsites elsewhere but don't want to put a lot of static calls to other random objects all over the place.

This technically "solves" T7477 (it changes "to" to "to + cc" for finding receivers) but doesn't yet implement proper behavior (with multiple receivers, for example).

Test Plan: Ran unit tests, which cover this pretty well. Additional changes will vet this more thoroughly.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7477

Differential Revision: https://secure.phabricator.com/D19948