Fix policy behavior of "slowvote.info" API method

Summary: Ref T13350. This ancient API method is missing modern policy checks.

Test Plan:

• Set visibility of vote X to "Only: epriestley".
• Called "slowvote.info" as another user.
• Before: retrieved poll title and author.
• After: policy error.
• Called "slowvote.info" on a visible poll, got information before and after.

Maniphest Tasks: T13350

Differential Revision: https://secure.phabricator.com/D20684