Retroactively populate Phriction mailKey column
Regardless of the adjustment issue, it's nice to populate this column anyway because there's no fundamental reason an object can't have mail sent about it without being saved first, even though it may not practically be possible in the codebase today.
- Ran storage upgrade, saw the column populate for older documents.
- Forced a couple of keys to bad values (too short or with "\0") and saw the migration fix them.
Reviewed By: btrahan
Differential Revision: https://secure.phabricator.com/D10804