Summary: Adding mail-keys; required for `PhabricatorApplicationTransaction` support.
Test Plan: Upgrade an old database with this patch, observe the matrix: {F42620}
Reviewers: wez, epriestley
Reviewed By: epriestley
CC: epriestley, aran
Maniphest Tasks: T2720
Differential Revision: https://secure.phabricator.com/D5852
