Summary: Ref T13065. Migrate "mailKey" and drop the column.
Test Plan: Ran "bin/storage upgrade", got a clean upgrade, saw migrated values in mail properties table.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13065
Differential Revision: https://secure.phabricator.com/D21634
