Add a "slowvote.poll.search" API method

Summary: Ref T13350. Add a modern "*.search" API method for Slowvote so "slowvote.info" can be deprecated with a reasonable replacement.

Test Plan: Used Conduit test console to call method, saw reasonable results.

Maniphest Tasks: T13350

Differential Revision: https://secure.phabricator.com/D20685