Summary: Ref T13054. Companion storage change for D19062.
Test Plan: Applied migration and adjustments. Viewed messages in Harbormaster; created them with `harbormaster.sendmessage`; processed them with `bin/phd debug task`.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13054
Differential Revision: https://secure.phabricator.com/D19063
