Migrate "SlowvotePoll" to modern mailkey storage
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.
Maniphest Tasks: T13682, T13065
Differential Revision: https://secure.phabricator.com/D21842