Summary: Ref T13641. Ref T13065. Migrate and drop the onboard "mailKey" column for Almanac Services.
Test Plan: Ran storage migration, got clean report, saw migrated value in mail properties table.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13641, T13065
Differential Revision: https://secure.phabricator.com/D21629
