R1:eeb009b4fa61

Modernize "mailKey" for Calendar Event

Summary: Ref T13065. Migrate "mailKey" and drop the column.

Test Plan: Ran "bin/storage upgrade", got a clean report, saw migrated data in mail properties table.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13065

Differential Revision: https://secure.phabricator.com/D21633