Add mail keys to ReleephRequests

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