Move Conpherence to "Z" monogram instead of "E"

Summary:
Ref T7439. Fixes T7438. This is only used in email right now.

Remain backward compatible.

Test Plan:

• Sent a message, saw a "Z" reply address.
• Checked the PHIDType.
• Grepped for "E" and 'E'.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7439, T7438

Differential Revision: https://secure.phabricator.com/D11956