Add PhabricatorSlowvoteQuery

Summary: Ref T603. Ref T2625. Start pushing Slowvote (the greatest app of all time) into the modern era.

Test Plan: Looked at vote detail. Used `V1` and `{V1}` embeds.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603, T2625

Differential Revision: https://secure.phabricator.com/D6444