R1:b1533e5468b1

Migrate "SlowvotePoll" to modern mailkey storage

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