Remove explicit "mailKey" from Owners packages

Summary:
Depends on D19426. Ref T13130. Ref T13065. While I'm making changes to Owners for "Ignore generated paths", clean up the "mailKey" column.

We recently (D19399) added code to automatically generate and manage mail keys so we don't need a ton of mailKey properties in the future. Migrate existing mail keys and blow away the explicit column on packages.

Test Plan: Ran migration, manually looked at the database and saw sensible data. Edited a package to send some mail, which looked good.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13130, T13065

Differential Revision: https://secure.phabricator.com/D19427