Migrate "SlowvotePoll" to modern mailkey storage


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.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13682, T13065

Differential Revision: https://secure.phabricator.com/D21842