R1:c3e6db6f0b32

Migrate Almanac Device "mailKey" to modern storage

Summary: Ref T13065. See similar changes attached to that task.

Test Plan: Ran migration, got a clean database state, saw mail keys populate in mail property table.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13065, T13641

Differential Revision: https://secure.phabricator.com/D21625