Summary:
Ref T13682. Ref T13065. Migrate and update "SlowvotePoll" to use more modern mailkey storage (in the mail application, rather than onboard).
Also make other minor modernization changes.
Test Plan: Created a new poll. Ran migration, verified mail key copied properly in database.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13682, T13065
Differential Revision: https://secure.phabricator.com/D21842
